Video report by ITV News Political Editor Robert Peston
The US has thrown its diplomatic weight behind Theresa May's retaliation against Moscow following the Salisbury nerve agent attack.
In a significant intervention following some concern over the stance of US President Donald Trump, the White House stated: "The United States stands in solidarity with its closest ally, the United Kingdom.
"The United States shares the United Kingdom's assessment that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter, and we support the United Kingdom's decision to expel Russian diplomats as a just response.
"This latest action by Russia fits into a pattern of behaviour in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes."
The comments came after the diplomatic drama shifted to the UN Security Council.
A showdown gathering at the world body saw Britain call on the international chemical weapons watchdog to verify its findings that Moscow is behind the Salisbury incident.
The UK's deputy UN ambassador, Jonathan Allen, told a special meeting of the Security Council that the Organisation for the Prohibition of Chemical Weapons had been asked to go over the British analysis of the attack.
In heated exchanges at the Security Council gathering, Russia strongly denied it was involved in the Salisbury incident, and the US offered Britain its full support.
US ambassador to the UN Nikki Haley said: "The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent. Dozens of civilians and first responders were also exposed.
"If we don't take immediate concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used. This is a defining moment."
The Russian permanent representative to the UN, Vassily Nebenzia, said: "We demand that material proof be provided of the allegedly found Russian trace in this high-resonance event.
"Without this, stating that there is incontrovertible truth is not something that we can take into account."
An Elysee source insisted Paris was fully behind Britain, telling the Press Association: "France's solidarity with the UK is unquestionable.
"President Macron denounced as early as Tuesday the Salisbury chemical attack as unacceptable and assured Prime Minister May the UK had France's full support. Both leaders will discuss the matter yet again this Thursday.
"Since the beginning of this week, the UK has briefed its allies thoroughly, and France in particular, that it was highly likely that Russia was responsible for the attack.
Earlier, May announced that 23 Russian diplomats will be expelled from the UK in the next week following the Kremlin's failure to meet a deadline to explain the nerve agent attack on a former spy.
It was one of a number of measures announced by the Prime Minister, who says the Russian state was culpable in the attempted murder of Sergei Skripal and his daughter Yulia in Salisbury.
On the other side of the Chamber, Labour leader Jeremy Corbyn was heckled for failing to condemn Russia directly.
The Russian embassy responded to the retaliation robustly, describing them as "unacceptable, unjustified and shortsighted".
What action has the Prime Minister taken?
The Prime Minister says the 23 Russian who will be forced to leave the country have been identified as "undeclared intelligence officers".
No members of the Royal Family nor politicians will attend the World Cup in Russia over the summer, while an invitation for the country's foreign minister Sergei Lavrov to visit the UK has been revoked.
Mrs May said Britain would make "full use" of existing powers to monitor and track those visiting the UK who could be engaged in activity threatening Britain's security.
This will include increased checks on private flights, customs and freight traffic, she said.
"We will freeze Russian state assets wherever we have evidence that they may be used to threaten the life or property of UK nationals or residents," said Mrs May.
Video report by ITV News correspondent Romily Weeks.
How did Russia respond?
In a statement the Russian Embassy said: "On March 14 Russian Ambassador Alexander Yakovenko was summoned to the Foreign and Commonwealth Office where he was informed that 23 diplomats were declared personae non gratae.
"We consider this hostile action as totally unacceptable, unjustified and shortsighted.
"All the responsibility for the deterioration of the Russia-UK relationship lies with the current political leadership of Britain."
Mrs May told MPs that it was "not in our national interest to break off all dialogue between the United Kingdom and the Russian Federation, but in the aftermath of this appalling act against our country this relationship cannot be the same".
NATO support for UK
Nato said the UK had briefed its North Atlantic Council on Wednesday about the Skripal attack.
A statement said: "Allies expressed deep concern at the first offensive use of a nerve agent on Alliance territory since NATO's foundation.
"Allies expressed solidarity with the UK, offered their support in the conduct of the ongoing investigation, and called on Russia to address the UK's questions including providing full and complete disclosure of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons.
"Allies agreed that the attack was a clear breach of international norms and agreements.
"Nato regards any use of chemical weapons as a threat to international peace and security."
Mr Corbyn was was widely heckled in the chamber for his response to the Salisbury attack in which he condemned the "appalling act of violence" before urging continued "robust dialogue" with Russia.
Mr Corbyn was also criticised by Tory MPs after saying it was a "matter of huge regret" that budgets to the diplomatic service had been cut..Mrs May told Mr Corbyn that there was a consensus on the situation - it just did not extend to the Labour leader.
He later faced shouts of "That's how you do it" after SNP Westminster leader Ian Blackford had insisted a "robust response to the use of terror on our streets" was needed, and pledged his party would work "constructively" with the Government.
Many Labour backbenches spoke in favour of the Prime Minister's action in relation to the nerve agent attack.
A spokesperson for Mr Corbyn said afterwards: "Clearly whoever carried out the attack is responsible for what was a completely heinous and reckless act."
Mr Corbyn failed to condemn Russia directly and does not accept the government version that Russia is to blame, with Mr Corbyn's spokesman adding: "There is a history between WMDs (Weapons of Mass Destruction) and intelligence which is problematic, to put it mildly.”
Mrs May was then asked about this statement during the debate, she said: "I am surprised and shocked at the statement that has been put out, and it is very clear from the remarks that have been made by backbenchers that they will be equally concerned by that."
Britons travelling to Russia urged to 'remain vigilant'
Travellers to Russia have been warned they could face "anti-British sentiment or harassment" as a result of the political tension between the UK and Moscow.
As Theresa May ordered the biggest expulsion of Russian diplomats since the Cold War, the Foreign Office urged Britons to "remain vigilant".
Thousands of England fans are expected to travel to Russia for the World Cup in June.
In its updated travel advice, the Foreign Office warned British nationals currently in Russia or due to travel in the coming weeks that "due to heightened political tensions between the UK and Russia, you should be aware of the possibility of anti-British sentiment or harassment at this time".
"You're advised to remain vigilant, avoid any protests or demonstrations and avoid commenting publicly on political developments," the advice said.
Counter-terrorism police, meanwhile, launched an investigation into the death of a Russian exile who was a close friend of Vladimir Putin critic Boris Berezovsky.
Scotland Yard said a man in his 60s was found at a home in Clarence Avenue, New Malden, south-west London on Monday and that the cause of his death is unexplained - but there was "no evidence to suggest a link to the incident in Salisbury".
World leaders, including Donald Trump and Angela Merkel, voiced their support for the UK as the deadline approached.