Russia's Foreign Ministry ordered the expulsion of 23 British diplomats from Russia on Saturday in a tit-for-tat response to Britain's decision to expel Russian envoys in connection with the poisoning of a former Russian double agent and his daughter on British soil.
The ministry also declared it was closing the British Consulate General in St. Petersburg and the British Council in Russia, in a step beyond the measures taken by Britain. The British Council is a cultural institute with artistic, language and educational programs.
The British diplomats have a week to leave, the Russian Foreign Ministry said, adding that its actions came "in response to the provocative actions of the British side and groundless accusations" against Russia over the Salisbury attack.
"The British side is warned that in case of further unfriendly actions against Russia, the Russian side reserves the right to take further retaliatory measures," the ministry said.
The United Kingdom's ambassador to Moscow, Laurie Bristow, was summoned to the ministry on Saturday morning to be told of Moscow's decision. The British Embassy in Moscow told CNN it had been given a list of 23 specific individuals that were to be expelled from Russia.
Relations between the two nations have deteriorated rapidly since the March 4 nerve agent attack on Sergei Skripal and his daughter Yulia in the English city of Salisbury. The pair remain critically ill in the hospital.
UK Prime Minister Theresa May on Wednesday gave the 23 Russian diplomats -- whom she described as undeclared intelligence officers -- a week to leave, as she accused the Russian state of being "culpable" for the attack in Salisbury.
Russia, which denies any involvement in the incident, condemned May's decision as unacceptable and vowed a swift response.
In comments to reporters after he left the Foreign Ministry, Bristow said the crisis had arisen because of "the attempted murder of two people using a chemical weapon developed in Russia and not declared by Russia." He said Britain must defend itself.
"We have no dispute with the Russian people, and a very large part of the work of my embassy here in Russia has been, is, to promote those links, those mutually productive links, between Russia and the United Kingdom," he said.
"But we will always do what is necessary to defend ourselves, our allies, and our values against an attack of this sort, which is an attack not only on the United Kingdom, but upon the international rules-based system, on which all countries, all countries including Russia, depend for their safety and security."
UK: Russia in 'flagrant breach' of law
Russia's response had been anticipated, a UK Foreign Office spokesperson said in a statement, adding that the UK National Security Council would meet early next week to discuss next steps.
"Our priority today is looking after our staff in Russia and assisting those that will return to the UK," the statement said.
"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 is Russia that is in flagrant breach of international law and the Chemical Weapons Convention."
The UK government was backed this week by allies France, Germany and the United States in its assessment that there was "no plausible alternative explanation" than that Russia was responsible for the nerve agent attack. UK officials believe the Skripals were exposed to a nerve agent known as Novichok that was developed in Russia.
UK Foreign Secretary Boris Johnson upped the stakes Friday when he said it was "overwhelmingly likely" that Russian President Vladimir Putin personally gave the order to use the nerve agent against the Skripals.
Kremlin spokesman Dmitry Peskov, speaking to CNN, slammed that remark.
"We have said on different levels and occasions that Russia has nothing to do with this story," he said. "Any reference or mentioning of our President is nothing else but shocking and unpardonable diplomatic misconduct."
Russia has insisted it is ready to cooperate in investigating the attack in Salisbury if Britain reciprocates by sharing the evidence it holds.
Glushkov murder investigation
In a dramatic twist Friday, London's Metropolitan Police said that the death of Russian businessman Nikolai Glushkov, who was found dead Monday in his London home, was now being treated as murder.
A post-mortem exam on the Russian exile, who had links to compatriots who died in mysterious circumstances in the UK, "gave the cause of death as compression to the neck," a police statement said. There's no evidence at this stage that his death and the attack on the Skripals are linked, it said.
Russia's Investigative Committee said it had launched its own criminal proceedings in connection with the "attempted murder of a Russian citizen, Yulia Skripal" in Salisbury and the "murder" of Glushkov in London.
UK-Russia relations have been fractious ever since the assassination of another former Russian spy, Alexander Litvinenko, in 2006.
A UK inquiry found that two Russian agents poisoned Litvinenko at a London hotel bar in 2006 by spiking his tea with highly radioactive polonium-210, and that Putin "probably approved" Litvinenko's killing. The Kremlin has always denied the accusation.
Sergei Skripal was sentenced to 13 years in prison in 2006 for spying for Britain, according to Russian state media accounts of the closed hearing.
Russian court officials at the time said he'd received at least $100,000 for his work for MI6, the British intelligence service. He was granted refuge in the UK after a high-profile spy exchange between the United States and Russia in 2010.