Britain's Prime Minister Theresa May speaks during the scheduled Prime Minister’s Questions in the House of Commons, London, Wednesday March 14, 2018.

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LONDON - Britain will expel nearly two dozen Russian diplomats, sever high-level bilateral contacts with Moscow and take both open and covert action against Kremlin meddling after the poisoning of a former spy, the prime minister said Wednesday, plunging U.K.-Russian relations into their deepest freeze since the Cold War.

Prime Minister Theresa May told lawmakers that 23 Russian diplomats who have been identified as undeclared intelligence officers have a week to leave the country.

May spoke after Moscow ignored a midnight deadline to explain how a nerve agent developed by the Soviet Union was used against Sergei Skripal, an ex-Russian agent convicted of spying for Britain, and his daughter Yulia. They remain in critical condition in a hospital in Salisbury, southwestern England, after being found unconscious on March 4.

May said Russia had provided no explanation, and "there is no alternative conclusion other than that the Russian state was culpable for the attempted murder of Mr. Skripal and his daughter."

She announced a range of economic and diplomatic measures, including the suspension of high-level contacts with Russia. An invitation for Russian Foreign Minister Sergey Lavrov to visit Britain has been cancelled, and May said British ministers and royals won't attend the soccer World Cup in Russia this summer.

READ MORE: The Latest on the poisoning of a former Russian spy in England

Investigators in protective suits work at the scene near the Maltings shopping centre in Salisbury, England, Tuesday, March 13, 2018. The use of Russian-developed nerve agent Novichok to poison ex-spy Sergei Skripal and his daughter makes it “highly likely” that Russia was involved, British Prime Minister Theresa May said Monday.

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May also said Britain would clamp down on murky Russian money and strengthen its powers to impose sanctions on abusers of human rights — though she gave few details.

"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, promising to use all possible legal powers against criminals and corrupt elites.

"There is no place for these people — or their money — in our country," she said.

May vowed to use law-enforcement and intelligence powers to disrupt Russian spying and "threats of hostile state activity." She said some of the measures "cannot be shared publicly for reasons of national security."

The Russian Embassy in London said the expulsion of diplomats was "totally unacceptable, unjustified and shortsighted."

"All the responsibility for the deterioration of the Russia-U.K. relationship lies with the current political leadership of Britain," it said in a statement.

Russian Ambassador Alexander Yakovenko said Britain's actions were "a provocation."

Some Russia experts said the measures announced by May were unlikely to make Russian President Vladimir Putin's government change its behaviour. She didn't expel Russia's ambassador or announce sanctions against any individuals.

Critics of the British government have long claimed that the U.K. is reluctant to act against Russia because London's property market and financial sector are magnets for billions in Russian money.

"There does not seem to be any real appetite so far to investigate the ill-gotten gains of the Russian elite that have been laundered through London," said John Lough, an associate fellow in the Eurasia program at the Chatham House think-tank.

"It is not clear to me that London's response will hit the Kremlin where it hurts."

Moscow refused to comply with Britain's demands that it explain how Novichok — a form of nerve agent developed by the Soviet Union during the Cold War — came to be used in Britain. Russia said the U.K. must first provide samples of the poison collected by investigators.

Putin spokesman Dmitry Peskov told reporters on Wednesday that Britain has offered "baseless accusations which aren't backed up by any evidence."

Some in Russia have suggested that the nerve agent could have come from another former Soviet country.

Lawmaker Vladimir Gutenev, a member of Russia's state commission for chemical disarmament, said Russia had scrapped its stockpile of Novichok.

"It is hard to say what may be happening in neighbouring countries," he was quoted as saying by the Interfax news agency.

Britain has sought support from allies in the European Union and NATO, including the U.S., in response to the use of an illegal chemical weapon on British soil. May's office said President Donald Trump told the prime minister the U.S. was "with the U.K. all the way."

On Wednesday, it also called for an urgent meeting of the U.N. Security Council to discuss the investigation.

European Council President Donald Tusk said Wednesday that the attack was "most likely" inspired by Moscow and announced he would put the issue on the agenda at an EU leaders' summit next week.

May said Russia's use of a chemical weapon was "an affront to the prohibition on the use of chemical weapons. And it is an affront to the rules-based system on which we and our international partners depend."

"We will work with our allies and partners to confront such actions wherever they threaten our security, at home and abroad," she said.

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Vladimir Isachenkov in Moscow, and Raf Casert in Brussels, contributed to this story.

News from © The Associated Press, 2018