Russia has expelled 23 British diplomats in a carefully calibrated retaliatory move against London, which has accused the Kremlin of orchestrating a nerve toxin attack on a former Russian double agent and his daughter in southern England.
Escalating a crisis in relations, Russia said it was also shutting down the activities of the British Council, which fosters cultural links between the two countries, and Britain's consulate-general in St Petersburg.
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The Russian foreign ministry said on Saturday the 23 British diplomats had one week to leave the country.
The move follows Britain's decision on Wednesday to expel 23 Russian diplomats over the attack in the English city of Salisbury which left former Russian spy Sergei Skripal, 66, and his daughter Yulia Skripal, 33, critically ill in hospital.
Moscow announced the measures on the eve of a presidential election which incumbent Vladimir Putin should comfortably win.
The foreign ministry said Moscow's measures were a response to what it called Britain's "provocative actions and unsubstantiated accusations". It warned London it stood ready to take further measures in the event of more "unfriendly steps".
British Prime Minister Theresa May said Britain would consider its next steps with its allies in the coming days.
"We will never tolerate a threat to the life of British citizens and others on British soil from the Russian government. We can be reassured by the strong support we have received from our friends and allies around the world," May told her Conservative Party's spring forum in London.
The Russian foreign ministry summoned the British ambassador, Laurie Bristow, to its headquarters on Saturday morning to inform him of the retaliatory measures.
Bristow told reporters afterwards that Britain had only expelled the Russian diplomats after Moscow had failed to explain how the nerve toxin had got to Salisbury.
Britain's foreign ministry said it had anticipated Russia's response and that its priority was to look after its staff in Russia and assist those returning home.
"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," it said in a statement.
Britain's National Security Council is due to meet early next week to consider London's next steps.
Russian foreign ministry spokeswoman Maria Zakharova told the Rossiya 24 TV channel on Saturday that the most likely source of the Novichok nerve toxin was Britain itself or the Czech Republic, Slovakia, Sweden or the US.
Those countries, not Russia, had been intensively testing the substance since the end of the 1990s, Zakharova said. The assertion could not be immediately verified.
The Swedish and Czech foreign ministers and the Slovak foreign ministry all separately rejected the Russian claim.