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.
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.
- UK-Russia tension has potentially catastrophic consequences
- 'No evidence' Russia shot down plane - Winston Peters
- Salisbury attack 'changes things', but Russia trade deal still possible - Jacinda Ardern
The move followed 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.
The foreign ministry said Moscow's measures were a response to what it called Britain's "provocative actions and groundless 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 alongside 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," Ms May said at the Conservative Party's spring forum in London.
The Russian foreign ministry summoned the British ambassador, Laurie Bristow, to a meeting on Saturday morning in central Moscow at its Stalin-era headquarters during which he was informed of the retaliatory measures.
Mr 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 its priority now was looking after its staff in Russia and assisting those that will return 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.
Russia's response was more robust than expected. The closure of the British Council's Moscow office will sever cultural ties, while that of the consulate-general in St Petersburg will end Britain's diplomatic presence in Russia's second city.
Britain has escalated a war of words with Russia over the incident in recent days. On Friday, British Foreign Secretary Boris Johnson said it was overwhelmingly likely that Russian President Vladimir Putin himself had made the decision to use a military-grade nerve toxin to strike down Skripal.
Britain, the US, Germany and France have jointly called on Russia to explain the attack, while US President Donald Trump has said it looks as if the Russians were behind it.