The United Kingdom will expel 23 Russian diplomats over the poisoning of a former spy on British soil, Prime Minister Theresa May said on Wednesday, adding they have a week to leave Britain.

Speaking in parliament, the prime minister accused the Russian government of "unlawful" use of force, and held it culpable for the attempted murder of Sergei Skripal and his daughter.

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.

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 that the Russian diplomats have been identified as undeclared intelligence officers

Britain will also revoke its invitation for a visit by Russian Foreign Minister Sergey Lavrov and will not be sending any senior officials or members of the royal family to this year's World Cup in Russia, May added.

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

May also said she has spoken to U.S. President Donald Trump, German Chancellor Angela Merkel and French President Emmannuel Macron in the last 24 hours in an attempt coordinate efforts to stand up for rules-based international order.

Britain has summoned Russia's ambassador to the Foreign Office in London, a government source said on Wednesday ahead of the statement by May.

Speaking to Sky News, Russia's ambassador to the U.K., Alexander Yakovenko, called Britain's current course of action in the case a "provocation" that is unrelated to the Salisbury incident.