Relations between London and Moscow have crashed to a post-Cold War low after an attack on an ex-Russian spy, the first known offensive use of a nerve agent in Europe since World War Two.
Russia expelled 23 British diplomats as ties continue to worsen between the two countries after Sergei Skripal and his daughter were targeted with a nerve agent on UK soil.
The move comes in retaliation for Britain's decision to kick out 23 Russians, who Prime Minister Theresa May says were spies working under diplomatic cover.
Those now named by Moscow must leave within a week.
TRT World's Kieran Burke reports.
On Saturday, Theresa May said that Russia's expulsion of 23 British diplomats "doesn't change the facts of the matter" of the poisoning of a former double agent in an English city.
Russia was "in flagrant breach of international law," she told her Conservative Party's spring forum, adding that Britain "will consider our next steps in the coming days".
"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," she said.
May blames Russia for the nerve agent attack on former spy Sergei Skripal and his daughter Yulia in the English city of Salisbury on March 4, which has left them both fighting for their lives.
She warned that Britain "will never tolerate a threat to the life of British citizens and others on British soil from the Russian government."
But she said Britain had "no disagreement with the Russian people".
TRT World's Simon McGregor-Wood reports from Moscow.
Russia expelled the British diplomats in a retaliatory move over British accusations that the Kremlin orchestrated a nerve toxin attack on a former Russian double agent and his daughter in southern England.
Earlier this week, Britain announced the expulsion of 23 Russian diplomats and suspension of high-level contacts over the poisoning.
Relations between London and Moscow have crashed to a post-Cold War low over the attack, the first known offensive use of a nerve agent in Europe since World War Two.
The Russian Foreign Ministry summoned the British ambassador, Laurie Bristow, to a meeting on Saturday morning in central Moscow at headquarters during which he was informed of the measures.
Bristow told reporters afterwards that the crisis had arisen after "the attempted murder of two people using a chemical weapon developed in Russia". 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.
TRT World's Dana Lewis reports.
War of words
Russia has complained that Britain has failed to provide any evidence of its involvement in the Salisbury attack and has said it is shocked and bemused by the allegations.
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 United States, 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.
A British policeman was also poisoned when he went to help them and remains in a serious but stable condition.
Russian investigators said on Friday they had opened a criminal investigation into the attempted murder of Yulia Skripal and offered to cooperate with British authorities.
Russia offered some cooperation to British authorities after the 2006 London murder of ex-KGB agent Alexander Litvinenko too.
Britain said the assistance in that case was not enough, and in 2016, a judge-led inquiry concluded that Putin had probably approved Litvinenko's murder, something Moscow denies.
Putin, a former KGB spy who is poised to win a fourth term in an election on Sunday, has so far only said publicly that Britain should get to the bottom of what has happened.