MOSCOW — Russia on Saturday announced it is expelling 23 British diplomats and threatened further retaliatory measures in a growing diplomatic dispute over a nerve agent attack on a former spy in Britain.

Britain’s government said the move was expected, and that it doesn’t change their conviction that Russia was behind the poisoning of ex-agent Sergei Skripal and his daughter in the English city of Salisbury. Prime Minister Theresa May said Britain will consider further retaliatory steps in the coming days alongside its allies.

The Russian Foreign Ministry ordered the 23 diplomats to leave within a week. It also said it is ordering the closure in Russia of the British Council, a government-backed organization for cultural and scientific cooperation, and is ending an agreement to reopen the British consulate in St. Petersburg.

The announcement followed Britain’s order last week for 23 Russian diplomats to leave the U.K. because Russia was not cooperating in the case of the Skripals, who were found March 4 poisoned by a nerve agent that British officials say was developed in Russia. They remain in critical condition and a policeman who visited their home is in serious condition.

Britain’s foreign secretary accused Russian President Vladimir Putin of personally ordering the poisoning of the Skripals. Putin’s spokesman denounced the claim.

Britain’s Foreign Office said Saturday that “Russia’s response doesn’t change the facts of the matter – the attempted assassination of two people on British soil, for which there is no alternative conclusion other than that the Russian State was culpable.”

The British Council said it was “profoundly disappointed” at its pending closure. The organization has been operating in Russia since the 1991 fall of the Soviet Union.

“It is our view that when political or diplomatic relations become difficult, cultural relations and educational opportunities are vital to maintain ongoing dialogue between people and institutions,” it said.

The Russian statement said the government could take further measures if Britain makes any more “unfriendly” moves.

Britain’s National Security Council will meet early this week to consider the next steps, May said.

Western powers see the nerve-agent attack as the latest sign of alleged Russian meddling abroad.

The poisoning has plunged Britain and Russia into a war of recrimination and blame.

British Ambassador Laurie Bristow, who was summoned to the Foreign Ministry in Moscow on Saturday to be informed of the moves, said the poisoning was an attack on “the international rules-based system on which all countries, including Russia, depend for their safety and security.”

“This crisis has arisen as a result of an appalling attack in the United Kingdom, the attempted murder of two people, using a chemical weapon developed in Russia and not declared by Russia at the Organization for the Prohibition of Chemical Weapons, as Russia was and is obliged to do under the Chemical Weapons Convention,” he said.

But Russian lawmaker Konstantin Kosachev blamed Britain for the escalating tensions. “We have not raised any tensions in our relations, it was the decision by the British side without evidence,” he said.

Kosachev, who heads the foreign affairs committee in the upper house of the Russian parliament, said “I believe sooner or later we will learn the truth and this truth will be definitely very unpleasant for the prime minister of the United Kingdom.”

Russian Foreign Ministry spokeswoman Maria Zakharova denied that Russia or the Soviet Union had ever developed Novichok, the class of nerve agent Britain says was used to poison the Skripals.