France has backed Theresa May's assessment that Russia is culpable for the attack in Salisbury and says it stands in solidarity with the UK.
Moscow has warned it will expel British diplomats "soon" after the Prime Minister announced the biggest expulsion of Russian embassy staff since the Cold War.
Mrs May and French President Emmanuel Macron discussed developments in the case in an early morning telephone call.
The talks came after reports of a lukewarm response from the French government but Paris later issued a statement saying there was "no other plausible explanation" for the poisoning.
Boris Johnson confirmed the UK will submit a sample of the nerve agent to the Organisation for Prohibition of Chemical Weapons (OPCW) for it to carry out its own tests.
Russian foreign minister Sergei Lavrov said expulsion of British diplomats would "definitely" happen.
The US threw its diplomatic weight behind the UK on Thursday, saying it "stands in solidarity with its closest ally".
A statement posted by the French Embassy after the call between the PM and Mr Macron said: "Since the start of the week, the UK has kept France closely informed of the information collected by the British investigators, and of the elements which show Russian responsibility in the attack.
"France shares the assessment of the United Kingdom that there is no other plausible explanation and expresses once again its solidarity regarding its ally."
A Downing Street spokesman said: "President Macron said that France completely shares the UK's assessment that there is no plausible explanation other than that Russia was responsible for the attack and he once again expressed his full support for the UK as a close and strong ally."
Mr Johnson said the UK's response means Russia's intelligence capabilities in the country had been "basically eviscerated" for decades.
The Foreign Secretary claimed Russian President Vladimir Putin wanted to send a message to any defecting Russians that "you're going to die".
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.
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".
Twenty-three Russian diplomats identified as undeclared intelligence officers have been given a week to leave the UK, in the largest mass expulsion since 31 were ordered out in 1985 following the defection of double agent Oleg Gordievsky.
Jeremy Corbyn drew criticism for his stance on the Salisbury incident after his spokesman said the history of the use 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.