The British Embassy in Moscow told CNN it had been given a list of 23 specific individuals that were to be expelled from Russian Federation.
The Russian Foreign Ministry expelled 23 British diplomats on Saturday in a retaliatory move over British punitive measures against Russia and the former Russian spy attack row.
Today Russia's Foreign Ministry said the 23 British staff would be expelled from Moscow within a week.
Russian Federation 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 UK Foreign Office said in a statement that "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".
The Russian statement said the government could take further measures if Britain takes any more "unfriendly" moves toward Russia.
The measures included the expulsion of 23 diplomats in a week, the freezing of Russian state assets in Britain, the suspension of all planned high-level bilateral contacts and boycotting the ministers and the Royal Family from attending the FIFA World Cup in Russia.
The pair remain in the hospital and are critically ill.
Russian Federation has consistently denied any culpability, accusing Britain of refusing to hand over samples of the poison used.
Mr Bristow, speaking to reporters after being summoned, said the United Kingdom would "always do what is necessary to defend ourselves, our allies and our values against an attack of this sort".
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.
It ordered the diplomats to leave within a week.
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.
On Friday, police said they were treating the March 12 death of exiled businessman Nikolai Glushkov as murder after a post-mortem found he died from "compression to the neck".
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Russia has now denied that any program under the name Novichok ever existed, despite the evidence presented two decades ago by the Russian scientist Vil Mirzayanov, who revealed its existence after becoming concerned it violated Russia's commitments to the Chemical Weapons Convention.
The UK's Foreign Minister Boris Johnson has said London would submit a sample of the Novichok nerve agent used in the former spy's poisoning to the Organisation for the Prohibition of Chemical Weapons (OPCW), a United Nations body.
Skripal betrayed dozens of Russian agents to Britain before being arrested in Moscow and later jailed in 2006.
In what seems like a unusual coincidence, police now say Nikolai Glushkov, a Russian former businessman, was murdered last week at his home just outside London.
British police are not linking his case with the Salisbury attack.
Russian court officials at the time said he'd received at least $100,000 for his work for MI6, the British intelligence service.