Prime Minister Theresa May has confirmed British officials will discuss the "next steps" against Russia after the Kremlin expelled 23 British diplomats in the Salisbury spy attack row.
She echoed a statement from the Foreign Office which said Moscow's measures were "anticipated" after British ambassador to Moscow, Laurie Bristow, was summoned to the Foreign Ministry to be informed of the response.
Labour leader Jeremy Corbyn meanwhile called for a "thorough investigation" into the nerve agent attack as he condemned its use of a Soviet developed poison as "barbaric".
The latest developments came as Scotland Yard announced it is treating the recent death of another Russian dissident in London as murder.
What have May and Corbyn said?
Mrs May used the start of a speech on the NHS to send a message out to Russia.
She said the UK will "never tolerate a threat to the life of British citizens and others on British soil from the Russian government" after declaring the Kremlin "in flagrant breach of international law and Chemical Weapons Convention".
She added: "I repeat today that we have no disagreement with the Russian people. Many Russians have made this country their home and those who abide our laws and make a contribution to our society will always be welcome."
Mr Corbyn, speaking a Labour conference held at Newcastle University, focused attention on Russian financial influence on Westminster.
He pledged to see that "Russian money be excluded from our political system", adding that Britain must "stop serving Russian crony capitalism".
The Labour leader condemned the nerve agent attack as "barbaric and beyond reckless" and said the Russian authorities "must be held to account on the basis of the evidence and our response must be both decisive and proportionate".
What other action has Russia taken in the tit-for-tat retaliation?
The Russian Foreign Ministry is also ordering the closure of the British Council in Russia and ending an agreement to reopen the British consulate in St. Petersburg.
The ministry warned it could take further measures if Britain takes any more "unfriendly" moves toward Russia following the poisoning of the ex-spy Sergei Skripal and his daughter with a nerve agent.
How did Britain respond?
The Foreign Office replied by saying the government "anticipated a response of this kind and the National Security Council will meet early next week to consider next steps".
Speaking outside the Foreign Ministry, Mr Bristow said: "This crisis has arisen as a result of an appalling attack in the United Kingdom, the attempted murder of two people using a chemical weapon developed in Russia".
He said that appeals to Vladimir Putin's government to explain how the nerve agent had arrived on UK soil had gone unanswered.
"We have no dispute with the Russian people...But we will always do what is necessary to defend ourselves, our allies and our values against from an attack of this sort," he added.
What action had Britain taken?
The UK expelled 23 Russian diplomats earlier this week to worsen one of the biggest rifts between London and Moscow in years following the attack on ex-spy Sergei Skripal and his daughter Yulia using the rare Russian Novichok nerve agent.
Foreign Secretary Boris Johnson directly accused Russian President Vladimir Putin over the attack, saying that it was "overwhelmingly likely" that personally ordered it.
How did Putin respond to Mr Johnson's accusation?
Russia has repeatedly denied that it had anything to do with the poisoning, which has left Mr Skripal and his daughter still fighting for their lives and a police officer who helped them seriously ill.
Mr Putin's spokesman Dmitry Peskov said: "We have said on different levels and occasions that Russia has nothing to do with this story. Any reference or mentioning of our president is nothing else but shocking and unpardonable diplomatic misconduct."
It has also rejected British claims that the Novichok agent was made in a former Soviet state, saying that other countries also had the knowledge and ability to manufacture the substance.
Is the latest death in London linked to the attack on the Skripals?
There has been no link found between the suspected murder on Nikolay Glushkov and the nerve agent attack on the Skripals.
The Russian businessman exiled in the UK, was found dead in his New Malden home, south-west London, on Monday.
Investigators initially said the 68-year-old's death was being treated as unexplained.
But a post-mortem examination on Thursday concluded that he was killed by compression to the neck, the Metropolitan Police confirmed.