Russia tonight warned Britain not to threaten a nuclear power as it refused to meet a midnight deadline for explaining a nerve agent attack on a double agent.
The extraordinary threat came as Theresa May signed up US President Donald Trump's support to back the UK "all the way" in the stand off with the Kremlin.
As the incendiary row grew rapidly, Kremlin spokeswoman Maria Zakharova was reported as saying in Moscow one should not threaten a nuclear power.
Russia has demanded to see samples of the Novichok substance found in Sergei Skripal's body before it will consider responding to May's midnight deadline.
May has been building support among world leaders since announcing last night Britain believed it "highly likely" Russia was to blame for the attack.
Before speaking to Trump tonight, May had already secured the backing of German Chancellor Angela Merkel and French President Emmanuel Macron.
Following the call with President Trump, Downing Street said: "The Prime Minister spoke to President Trump earlier this afternoon to update him on the ongoing investigation into the Salisbury incident.
"The Prime Minister set out the conclusion reached by the UK Government that it was highly likely that Russia was responsible for the attack against Sergei and Yulia Skripal.
"President Trump said the US was with the UK all the way, agreeing that the Russian Government must provide unambiguous answers as to how this nerve agent came to be used."
In a series of incendiary tweets tonight, Russia's London embassy said: "Moscow will not respond to London's ultimatum until it receives samples of the chemical substance to which the UK investigators are referring.
"Britain must comply with the Chemical Weapons Convention which stipulates joint investigation into the incident, for which Moscow is ready.
"Without that, there can be no sense in any statements from London. The incident appears to be yet another crooked attempt by the UK authorities to discredit Russia.
"Any threat to take 'punitive' measures against Russia will meet with a response. The British side should be aware of that."
Earlier Trump told reporters outside the White House: "Well it sounds to me, I'm speaking to Theresa May today. It sounds to me like it would be Russia based on all of the evidence they have.
"I don't know if they've come to a conclusion, but she's calling me today."
Following the call with Merkel, Downing Street said: "They discussed the pattern of aggressive Russian behaviour and agreed it would be important to act in unison with allies to counter it.
"Chancellor Merkel condemned the attack and said she stood in full solidarity with the UK.
"They agreed that the international community should coordinate closely as the investigation developed and in the wake of Russia's response."
Senior MEP Guy Verhofstadt offered EU support and said: "We stand shoulder to shoulder with the British people.
''It must be made clear that an attack against one EU & NATO country is an attack on all of us."
It comes as the clock is ticking on the midnight deadline May has given for the Russians to explain how their nerve agent was used in an attempted murder on the sleepy streets of Salisbury.
British officials are drawing up a plan to hit back hard at Russia if - as expected - they do not have an explanation.
The Government is expected to draw up a list of tough sanctions that will target the Kremlin and Putin's cronies, freezing assets and stopping them from travelling to the UK .
They are also expected to expel many of Russia's diplomats - although probably not break off diplomatic relations altogether - with the country.
British officials and the Royal family are expected to boycott the football world cup in Russia later this year.
But Britain will need to get support from the country's UN and Nato allies in order to impose sanctions that will really bite and be felt by the Kremlin.
The PM yesterday dramatically pointed the finger at Putin for the poison attack saying it was'highly likely' it was ordered by the country.
The Prime Minister said the facts increasingly suggested Russia was behind the apparent 'hit' on double-agent Sergei Skripal in Salisbury.
Branding the attack a "reckless and despicable act", May said the substance used was a 'military grade' agent Moscow has produced.
May said the government would not accept such an attempt to "murder innocent civilians on our soil".
Foreign Secretary Boris Johnson said allies are mobilising to support the UK and hit back at Russia.
Speaking today he said: "I've been very encouraged so far by the strength of the support that we are getting.
"I think in particular from President Macron of France, I talked to Sigmar Gabriel my German counterpart, and from Washington where Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behaviour, increasingly disruptive behaviour, malign behaviour by Russia, the reckless use of chemical weapons, the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury.
"I've been encouraged by the willingness of our friends to show support and solidarity."
Home Secretary Amber Rudd oversaw an emergency Cobra committee into the crisis this morning.
She said: "The Russians have started responding. The Prime Minister has been very clear that they have until midnight tonight to satisfy her requests.
"Until then we will wait and see what they have put forward.
"I know that international allies have begun to rally their support and make comments publicly but at the moment what we are doing is awaiting the Russian response before stepping up and responding as the Prime Minister has said we will."
Whitehall sources said yesterday they were accelerating their offensive cyber programme and could hit select targets for a specific effect.
It is understood this could see a specialist cyber unit deployed in the UK to attack Kremlin computer networks spewing Russian propaganda and trolling factories spreading fake news.
Together with Russia's previous actions and tactics, including the killing of Alexander Litvinenko, the UK authorities yesterday concluded it was 'highly likely' to be involved in the episode.
In a tough statement updating MPs after a meeting of the National Security Council, May raised the prospect of significant retaliation - making clear that the UK is already consulting Nato and other allies.
"It is now clear that Mr Skripal and his daughter were poisoned with military grade nerve agent of a type developed by Russia," May said. "This is part of a group of nerve agents known as novichok."
Another option is for the Government to implement a British version of the US's Magnitsky Act, which lists Russians involved in corruption and human rights abuses, banning them from entering the country.
There could be fresh sanctions against senior figures in the Putin regime, either with the EU or unilaterally.
General Sir Chris Deverell, commander of Joint Forces Command, has revealed how the UK has a specialist unit which is dedicated to "offensive cyber" run jointly by the MoD and GCHQ.
So far it has worked on Islamic State but this could be expanded towards Russia.
In an interview with the Mail last week, he said the military could hit back at disinformation spread by Russian trolling factories.
He said: "There are two ways you could respond. One is putting your own messages out to compete with the messages that actors like that are sending. And the other is with a cyber-attack.
"Whether or not you could use cyber as a weapon would depend upon the specific circumstances and the law." He said that there was a specific capability in which troops tackle mistruths spread by enemies.
Russia's ambassador was summoned to the Foreign Office at 3.45pm yesterday for a "cool but firm" meeting with Boris Johnson. There was no handshake between the politicians as Johnson outlined the "outrage" felt by the British people.
She added: "Based on the positive identification of this chemical agent by world leading experts at the laboratory at Port Down, our knowledge that Russia has previously produced this agent and would still be cap able of doing so, Russia's record of conducting state sponsored assassinations, and our assessment that Russia views defectors as a legitimate target for assassination the government has concluded that it is highly likely Russia was responsible for the act against Sergei and Yulia Skripal."
May said Boris Johnson had summoned the Russian ambassador in London yesterday afternoon and informed him of the findings.
The Kremlin was given a deadline of midnight to respond to the evidence and the government could outline its 'detailed' retaliation later today.
"Should there be no credible response we will conclude that his action amounts to an unlawful use of force by the Russian state against the United Kingdom, and I will come back to this House and set out the full range of measures that we will take in response,' she said.
"This attempted murder using a weapons-grade nerve agent on a British town was not just a crime against the Skripals, it was an indiscriminate ad reckless act against the United Kingdom, putting the lives of innocent civilians at risk," she said.
"And we will not tolerate such a brazen attempt to murder innocent civilians on our soil."
But the Russian Foreign Ministry showed little sign of readiness to explain itself, immediately deriding May's remarks as a "circus show".
The ministry appeared to reference May's address to MPs with a video of a snow covered traffic jam, accompanied by the hashtag Highly Likely Russia, attempting to make light of the poisoning.
A clip of vehicles struggling in the snow as the Beast from the East was posted by the government branch, suggesting the extreme weather was Russia's fault.
And the video finishes with a penguin playing in snow, with the caption, "At least penguin enjoys it."
US Secretary of State Rex Tillerson backed Britain - but hours later it emerged that Trump has sacked him.
Tillerson said: "We have full confidence in the UK's investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week.
"There is never a justification for this type of attack - the attempted murder of a private citizen on the soil of a sovereign nation - and we are outraged that Russia appears to have again engaged in such behaviour.
"From Ukraine to Syria - and now the UK - Russia continues to be an irresponsible force of instability in the world, acting with open disregard for the sovereignty of other states and the life of their citizens.
"We agree that those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences.
"We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses."
Counter-terrorism police and intelligence officers are thought to have presented compelling evidence at the NSC meeting that Moscow ordered the hit in Salisbury over a week ago.
The nerve agent - Novichok - that contaminated the victims was so secret that it was never known to have been used until now.
Former Tory leader Iain Duncan Smith branded Russia a "rogue state" and demanded the "most severe" response.
He said: "If we appease a country like this, then we should expect even worse."
Commons Foreign Affairs Committee chair Tom Tugendhat said the use of nerve gas was a "war like act".
Home Affairs Committee chair Yvette Cooper said a cross-party stand was needed against Russian aggression.
In an interview with the Evening Standard earlier, Home Secretary Amber Rudd pointed out that Putin had jibed in a Russian TV interview about not being able to forgive "betrayal".
She said: "I'm not going to enter into a kind of great big tit-for-tat with them, which is what they are longing I'm sure for us to do.
"Because when attribution comes we have to be absolutely cool-headed about it. Other people can carry on making their comments.
"I think that the general public are wise enough to take a dim view of that kind of childish joshing."
May hinted that the prospect of pulling the England team out of the football World Cup was not being considered.
Asked about the team, she suggested officials could boycott the event but did not go further.
The dramatic moment in the House of Commons came after Public Health England stepped up their response by warning members of the public in Salisbury to wash their clothing and possessions – seven days after Skripal and his daughter Yulia fell ill.
Three Cabinet ministers - including Boris Johnson - are understood to have been be privately pushing the PM to respond with 'tough action' after claims Britain's response to the 2006 murder of Russian ex-spy Alexander Litvinenko had not been enough.
Ministers, spy chiefs, police and the head of the military were at the talks today on the progress made in the investigation.
Military scientists at the Porton Down research unit expect to be able to say beyond doubt that the rare nerve toxin was devised in a laboratory in Moscow.
The PM is thought to have canvassed views from Foreign Office officials and Air Chief Marshal Sir Stuart Peach on the appropriate response if the evidence proves conclusive.
Possibilities include "full spectrum" retaliation across diplomatic, economic and military fronts such as the immediate expulsion of senior Russian diplomats and spies, potentially cancelling Kremlin-linked oligarchs' visas, and asset freezes and travel bans.
The Government is also expected to meet MPs "half way" on implementing a British version of the US's "Magnitsky Act" which lists Russians involved in corruption and human rights abuses who are banned from entering America.
Senior Whitehall sources said the Government could also plan long-term consequences such as boosting Britain's military deployments in eastern Europe.
Further options include covert consequences for Moscow that will never be revealed such as an offensive cyber-attack.
Marina Litvinenko, whose husband Alexander died in London after being poisoned, said the Government had promised her such a crime would not be repeated.
"Unfortunately, it happened again. It means something was not done," she said.
The PM's spokesman said senior British officials have been speaking with top officials from Britain's Nato allies and further talks are expected in the coming days.
Asked specifically if she was using "Nato or UN" language in her statement, he added: "That is not an Article 5 type statement."
He also signalled that the Government could back amendments being tabled by MPs to a Bill going through Parliament to beef up the power to impose sanctions on cronies of Putin using dirty money to love the high life in London.
A group of backbench MPs want the Government to bring in a Magnitsky Law, which has already been introduced in other countries including the US.
The PM's spokesman said the Government has already passed legislation to beef up the sanctions regime.
But he added that May "is looking to build the broadest possible consensus and conversations will take place with those who have tabled the amendment".
Nato Secretary General Jens Stoltenberg said the organisation was concerned by the incident and that it was backing Britain.
He said: "The United Kingdom has concluded that Sergei Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia.
"And Prime Minister Theresa May stated today that it is highly likely that Russia was responsible for the act. The use of any nerve agent is horrendous and completely unacceptable.
"The UK is a highly valued ally, and this incident is of great concern to Nato. Nato is in touch with the UK authorities on this issue."
While Russia is not a Nato member, France is, and Theresa May spoke to her counterpart President Emmanuel Macron yesterday to keep him in the loop.
A Downing Street spokesman said: "The Prime Minister spoke to President Macron of France to update him on the latest situation regarding the incident in Salisbury on 4 March.
"She outlined the conclusion reached by the Government that it was highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.
"They discussed the wide pattern of aggressive Russian behaviour and agreed that it would be important to continue to act in concert with allies to address it.
"President Macron condemned the attack and offered his solidarity with the UK.
"They agreed that the French and British governments should co-ordinate closely as the investigation developed and following Russia's response."