LONDON (Reuters) - Britain will expel 23 Russian diplomats in response to a nerve agent 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 told parliament Britain would also freeze Russian state assets wherever there was evidence of a threat and downgrade its attendance at the soccer World Cup this summer.

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 a critical condition.

May has said the pair 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.

Russia has denied any involvement, and May told parliament Moscow had provided no credible explanation for the attack.

“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,” she said.

“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 said no ministers or members of the royal family would attend the World Cup in Russia.

Russia said Britain should expect retaliation for its actions.