Russian Federation on Saturday announced it is expelling 23 British diplomats and threatened further retaliatory measures in a growing diplomatic dispute over a nerve agent attack on a former spy in Britain.
Britain's Foreign Office said in a statement on Saturday that the country's National Security Council would meet early next week to consider its next steps.
The tit-for-tat expulsion follows after the poisoning of a former Russian spy and his daughter in London on March 4.
Russian ex-spy Sergei Skripal, 66, and his daughter Yulia Skripal, 33, remain critically ill in hospital, after they were found unconscious on a bench in Salisbury, Wiltshire.
The Foreign Office said in a statement that it had expected the Russian retaliation, which includes closing the British consulate in St. Petersburg and barring cultural organization the British Council.
"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."
Russian Foreign Ministry spokeswoman Maria Zakharova denied that Russia or the Soviet Union had ever developed Novichok, the class of nerve agent that Britain says was used to poison the Skripals.
The Russian Ministry of Foreign Affairs said in a statement Saturday that 23 British diplomats are "persona non grata" and must leave the country within a week.
It also said it would close the British Council in Russian Federation, which promotes cultural ties between the nations, and the British Consulate in St Petersburg.More news: Iran, US to hold talks on 2015 nuke deal
Britain's foreign secretary accused Russian President Vladimir Putin of personally ordering a nerve agent attack in Britain, ratcheting up tensions on Friday (Saturday NZ Time) in an increasingly global showdown over alleged Russian meddling overseas.
The Foreign Ministry said Moscow's measures were a response to what it called Britain's "provocative actions and groundless accusations".
"This follows the action we have taken, alongside other measures, to dismantle the Russian espionage network operating in the United Kingdom as a outcome of the attempted assassination of two people here in Britain using a nerve agent". On Saturday, Russia's envoy to the Organisation for the Prohibition of Chemical Weapons (OPCW), Alexander Shigulin alleged to the news agency Interfax the "most likely" source of the nerve agent was Britain or the United States.
They were ordered to leave over the incident on 4 March which the United Kingdom government has blamed on Russian Federation - but which Russian Federation denies.
But Russian lawmaker Konstantin Kosachev blamed Britain for the escalating tensions. "Russian Federation did neither, therefore we announced certain steps". A police officer was also harmed and remains in a serious condition.
The global chemical weapons watchdog says the class of nerve agents used in the Skripal attack has never been declared by any of its member states.
Putin's spokesman denounced the claim as "shocking and inexcusable".
"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 worldwide rules-based system on which all countries, including Russian Federation, depend for their safety and security", Bristow told reporters.
She said the expulsion is a reflection that this is not the only time Russian Federation has acted against the U.K. In addition, new legislative powers will be enacted to combat hostile state activity, including a power to detain suspects at the border.