Having warned it would retaliate proportionately, this morning Russia did just that when it expelled 23 British diplomats - the same number as the UK kicked out a few days earlier as punishment for Moscow's alleged poisoning of a former double agent. It also ordered the closure of the UK consulate in St Petersburg and the Moscow British Council, a cultural and educational organization.

Russia’s Ministry of Foreign Affairs summoned the British ambassador to Moscow and told him that the measures are “in response to the provocative actions of the British side and the unsubstantiated accusations” against Russia, the ministry said. Russia gave the British diplomats one week to leave. “If further actions of an unfriendly nature are taken against Russia, the Russian side reserves the right to take other retaliatory measures,” the ministry said.

British ambassador to Russia, Laurie Bristow, attends a meeting at the Russian foreign ministry

A spokeswoman for the U.K. Foreign Office said that Britain had anticipated Moscow’s response.

“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 spokeswoman said but added that "we continue to believe it is not in our national interest to break off all dialogue between our countries but the onus remains on the Russian state to account for their actions."

She said that the UK Foreign Office said the National Security Council would meet early next week to consider the next steps.

The order to close the British Council ends nearly 60 years of its work in Russia as the U.K.’s international organization for culture and education, Bloomberg reported. It opened offices in Moscow under a 1959 agreement with the Soviet Union and expanded to 15 Russian cities after the 1991 collapse of the Communist state. Its presence gradually reduced amid mounting political confrontation between the U.K. and Russia, which also disputed the legal basis for the council’s presence in the country. In 2008, Russia ordered the council to close all its offices except the Moscow headquarters as part of retaliation for the U.K.’s expulsion of diplomats over the radioactive poisoning of former security-service officer Alexander Litvinenko in 2006. A U.K. public inquiry concluded in 2016 that Putin “probably” approved the killing.

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Diplomatic relations between London and Moscow collapsed to post-Cold War lows following the poisoning of Sergei Skripal, a former Russian military intelligence agent living in the UK, and his daughter Yulia earlier this month with a rare nerve agent manufactured during the Soviet era.

As reported last night, UK's foreign secretary Boris Johnson escalated the diplomatic clash on Friday by accusing Vladimir Putin of personally ordering the poisoning. Boris Johnson said that it was “overwhelmingly likely” that the decision to carry out an assassination attempt was made by the Russian president.

Johnson said: “Our quarrel is with Putin’s Kremlin, and with his decision — and we think it overwhelmingly likely that it was his decision — to direct the use of a nerve agent on the streets of the UK, on the streets of Europe for the first time since the second world war.” The Kremlin responded that his comments were “unforgivable” and “shocking”, while Downing Street declined to remark on the direct accusation.

Russia has denied any involvement in the attack on Mr Skripal, who was convicted of spying for Britain, then sent to the UK in a prisoner exchange in 2010. But Russia has also sent unambiguous messages on state TV about the fate of traitors.

The Russian foreign ministry said the UK’s accusations of Russian state involvement in the poisoning groundless. It said Laurie Bristow, the UK Ambassador to Russia, had been told the expulsions were ordered “in response to the provocative actions of the British side and the unsubstantiated accusations” against the country.

On Thursday, the U.S. joined the U.K., France and Germany in condemning the attack as “an assault on U.K. sovereignty,” saying it constituted a breach of international law and calling on Russia to explain its role in the poisoning in Salisbury, England.

President Donald Trump, French President Emmanuel Macron and German Chancellor Angela Merkel shared the U.K.’s assessment that it was highly likely that Russia was responsible for the attack—the first use of a nerve agent in a North Atlantic Treaty Organization country. NATO Secretary-General Jens Stoltenberg has condemned the use of the poison, saying it “has no place in a civilized world.”

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Saturday’s retaliation by Moscow also comes after the Trump administration issued its first sanctions against Russia for meddling in the 2016 U.S. presidential elections, as well as for its role in the NotPetya cyberattack and in the nerve-agent poisoning.

Russia has denied any interference in the U.S. election, while Russian President Vladimir Putin, who runs for re-election Sunday, has steered an increasingly confrontational course with the West. The Kremlin previously expelled some U.S. diplomats in 2017 after Congress passed a Russian sanctions bill.

Moscow has yet to retaliate against the latest round of US sanctions.