Theresa May is under increasing pressure from her own ministers to retaliate against Russia for what one politician says, if proven, was a "brazen act of war" in using nerve gas on the streets of Britain.
Gavin Williamson, the Defence Secretary, said the Government was being "pushed around" by Russia and "we have to change the way we deal with it", amid calls for fresh sanctions against Moscow and the expulsion of Russian diplomats.
Boris Johnson, the Foreign Secretary, had already threatened sanctions if Russia was found to be behind the attack that left Sergei Skripal, a former double agent, and his daughter Yulia in comas and Nick Bailey, a police sergeant, in intensive care.
Yesterday Johnson repeated his insistence that Britain would respond "robustly" against any country found to be responsible. Downing Street said it could not make a diplomatic move until it was proved who was behind the attempted murders.
A Whitehall source said: "There is a process to go through. These things have to be done in the right order."
Amber Rudd, the Home Secretary, told the Commons there would be consequences for the perpetrators, who would be brought to justice "whoever they are and wherever they may be".
But Tom Tugendhat, the Conservative chairman of the Foreign Affairs Select Committee, made it clear to Mrs May that "the first duty of government is to protect the British people - I think using nerve agents on British streets really does demand a response".
Williamson, who has claimed in the past that Russia was ready to kill "thousands and thousands" of British people, said yesterday it was becoming an "ever-greater threat", particularly to its closest neighbours.
He said: "Russia's being assertive, Russia's being more aggressive, and we have to change the way that we deal with it because we can't be in a situation in these areas of conflict where we are being pushed around by another nation."
If Russia is behind this, this is a brazen act of war, of humiliating our country.
Sir Edward Leigh, formerly chairman of the Public Accounts Committee, said the circumstantial evidence against Russia was "very strong" and told the Commons: "If Russia is behind this, this is a brazen act of war, of humiliating our country."
Rudd said: "There will come a time for attribution and there will be, then, consequences and there will be further information that follows."
She said the investigation must be based on "facts, not rumour", but added: "The use of a nerve agent on UK soil is a brazen and reckless act. This was attempted murder in the most cruel and public way.
"People are right to want to know who to hold to account. We are committed to doing all we can to bring the perpetrators to justice - whoever they are and wherever they may be."
Tugendhat, a former Army officer, said it now seemed likely that Skripal's wife Lyudmila and son Alexandr had both been murdered.
The circumstances of their deaths, in 2012 and 2017, have been disputed, with some reports claiming they died in car crashes, others stating health reasons.
Pointing to threats made against Colonel Skripal and the similarity of the attack to a fatal one on Alexander Litvinenko, the Russian dissident, ugendhat said: "We are beginning to see not only a very strong pattern but a very strong centre to that pattern - and that centre appears very strongly to be the Kremlin."
Johnson said: "I think we have to wait for the outcome. But if, and it's still a big if, it turns out this is the agency of another state attempting to kill people on UK soil, then of course the UK will respond robustly."