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 yesterday, 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,” she said.