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 England.
Escalating a crisis in relations on Saturday, 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.
The Russian Foreign Ministry said it was giving the 23 British diplomats one week to leave the country.
The move, which was tougher than expected, followed Britain's decision on Thursday 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. Putin has cast his country as a fortress besieged by hostile Western powers with him as its defender, and state media is likely to portray the anti-British move in that context.
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".
Relations between London and Moscow have crashed to a post-Cold War low over the Salisbury attack, the first known offensive use of a nerve agent in Europe since World War II.
The Russian Foreign Ministry summoned British ambassador Laurie Bristow to a meeting on Saturday morning at its Stalin-era headquarters during which he was informed of the 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 Britain, he said.
"We will always do what is necessary to defend ourselves," the ambassador told reporters.
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.
Russian news agencies cited politicians in Russia's upper house of parliament as welcoming the move to close the British Council, alleging it had been used as a cover by British spies.
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.
Russia has said is open to cooperation with Britain, but has refused Britain's demands to explain how Novichok, a nerve agent developed by the Soviet military, was used against the Skripals.
Skripal, a former colonel in the GRU who betrayed dozens of Russian agents to British intelligence, and his daughter have been critically ill since March 4, when they were found unconscious on a bench.
