Britain will expel 23 Russian diplomats in response to a nerve toxin attack on a Russian former double agent in southern England, Prime Minister Theresa May said on Wednesday, adding it was the biggest single expulsion in over 30 years.

May said Britain would also introduce new measures to strengthen defences against hostile state activities, freeze Russian state assets wherever there was evidence of a threat and downgrade its attendance at the soccer World Cup in Russia this summer.

Russia, which has repeatedly denied any involvement in the nerve agent attack, said Britain should expect retaliation for its actions.

Former spy Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench in the city of Salisbury on March 4 and remain in hospital in 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.

"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 over 30 years and would degrade Russian intelligence capabilities in Britain for years to come.

"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," May 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.

On the soccer World Cup, she said no ministers or members of the British royal family would attend.