The US has thrown its diplomatic weight behind the UK as Britain braces itself for Russian retaliation after Theresa May hit back at Moscow over the Salisbury nerve agent attack.
As the Kremlin made it clear it is readying its response to the biggest expulsion of its embassy staff since the Cold War, the White House came down firmly on Britain's side.
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."
Announcing sanctions in the House of Commons, the PM said the attack on ex-spy Sergei Skripal and his daughter Yulia amounted to "an unlawful use of force by the Russian state against the United Kingdom".
Mrs May announced the suspension of high-level contacts with Russia, including a boycott of this summer's World Cup by Government ministers and members of the royal family.
And she said Russian state assets will be frozen "wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents".
Russia's Ministry for Foreign Affairs branded Mrs May's statement as "an unprecedentedly crude provocation that undermines the foundations of a normal interstate dialogue between our countries".
The 23 Russian diplomats identified as undeclared intelligence officers have been given a week to leave, in the largest mass expulsion since 31 were ordered out in 1985 following the defection of double agent Oleg Gordievsky.
Asked if the UK should be embarrassed it had allowed "bad people" to park money in London, Security Minister Ben Wallace told BBC2's Newsnight: "I think we all collectively in the body politic have to take responsibility for that.
"We have allowed the City of London's reputation as a centre for world finance to be exploited by some pretty nasty individuals who have used illicit money flows from around the world to come here, either to harbour it, or to clean it, or to just move it around, or invest it."
Jeremy Corbyn drew criticism for his stance on the Salisbury incident after his spokesman said the history of information from UK intelligence agencies is "problematic" and refused to say that the Labour leader accepted the Russian state was at fault.
The spokesman's comments prompted Labour backbencher John Woodcock to table an Early Day Motion "unequivocally" accepting the "Russian state's culpability" for the attack, and supporting "fully" the statement made by Mrs May in the Commons.
Labour's shadow security minister Nick Thomas-Symonds told BBC2's Newsnight: "I don't think it's about flawed intelligence. We have great confidence in the work the security services do.
"It's a distinction, obviously, between the interpretation that politicians can make of intelligence and intelligence itself."