Britain will expel 23 Russian diplomats over a nerve toxin attack on a Russian ex-spy in England, Prime Minister Theresa May says, describing the attack as an unlawful use of force by Russia against the United Kingdom.

May says Britain would also introduce new measures to strengthen defences against hostile state activities and freeze Russian state assets wherever there was evidence of a threat.

She also says no ministers or members of the British royal family would attend the football World Cup in Russia.

Britain's Prime Minister Theresa May speaks during the scheduled Prime Minister's Questions in the House of Commons. Photo / AP
Britain's Prime Minister Theresa May speaks during the scheduled Prime Minister's Questions in the House of Commons. Photo / AP

Russia, which denies any involvement in the attack, has called the measures announced by May "unacceptable, unjustified and short-sighted" and warned Britain to expect retaliation.

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Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench in the genteel city of Salisbury on March 4 and remain in hospital in a critical condition. A police officer was also harmed and remains in a serious condition.

May has said the Skripals were attacked with Novichok, a Soviet-era military-grade nerve agent. She had asked Moscow to explain whether it was responsible for the attack or had lost control of stocks of the highly dangerous substance.

"Their response demonstrated complete disdain for the gravity of these events," May said in a statement to parliament on Wednesday.

"They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance.

"There is no alternative conclusion, other than that the Russian state was culpable for the attempted murder of Mr Skripal and his daughter, and for threatening the lives of other British citizens in Salisbury, including Detective Sergeant Nick Bailey.

"This represents an unlawful use of force by the Russian state against the United Kingdom."

May said the expulsion of the 23 diplomats, identified as undeclared intelligence officers, was the biggest single expulsion for more than 30 years and would degrade Russian intelligence capabilities in Britain for years to come.

The expelled Russian diplomats have one week to leave Britain, May said, before listing other measures.

Investigators in protective suits work at the scene in the Maltings shopping centre in Salisbury. Photo / AP
Investigators in protective suits work at the scene in the Maltings shopping centre in Salisbury. Photo / AP

"We will freeze Russian state assets wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents," she said.

She also said new legislative proposals would be urgently developed to counter any threat from a hostile state.

"This will include the addition of a targeted power to detain those suspected of hostile state activity at the UK border," May said.

British authorities would make use of existing powers to enhance efforts to monitor and track the intentions of those travelling to the UK who could be engaged in activities that represented a security threat.

"We will increase checks on private flights, customs and freight," she said.

She also threatened action against those she described as "serious criminals and corrupt elites", adding: "There is no place for these people, or their money, in our country."

May said Britain would revoke an invitation to Russian Foreign Minister Sergei Lavrov to visit the country and suspend all planned high level bilateral contacts between London and Moscow.

Police close the entrance to the car park of a supermarket after finding an abandoned car near where former Russian double agent Sergei Skripal and his companion were found critically ill. Photo / AP
Police close the entrance to the car park of a supermarket after finding an abandoned car near where former Russian double agent Sergei Skripal and his companion were found critically ill. Photo / AP

Britain was not alone in confronting Russian aggression, May said, adding she had discussed the Salisbury attack with US President Donald Trump, German Chancellor Angela Merkel and French President Emmanuel Macron.

Donald Tusk, European Union council president, earlier voiced support for Britain and stood ready to put the attack on the agenda of a council meeting next week.

Britain asked for an urgent meeting of the United Nations Security Council to update members on the attack.

May also said London had notified the Organisation for the Prohibition of Chemical Weapons about the use of the nerve agent.