The Prime Minister made an appeal for European solidarity at the EU Council in Brussels amid a diplomatic stand-off with Vladimir Putin over the poisoning of ex-Russian spy Sergei Skripal and his daughter Yulia.
Donald Tusk, the EU Council President, said the 28 leaders agree it is “highly likely” the Russian state is responsible for the attack as there is “no other plausible explanation”, but it remains unclear if the bloc will back joint action.
In a further development it emerged the European Union was recalling its ambassador to Moscow “for consultations”, although the move was not a formal sanction.
The leaders issued a joint text backing Britain, saying: “The European Council condemns in the strongest possible terms the recent attack in Salisbury, expresses its deepest sympathies to all whose lives have been threatened and lends its support to the ongoing investigation.
“It agrees with the United Kingdom Government’s assessment that it is highly likely that the Russian Federation is responsible and that there is no plausible alternative explanation. We stand in unqualified solidarity with the United Kingdom in the face of this grave challenge to our shared security.
“The use of chemical weapons, including the use of any toxic chemicals as weapons under any circumstances, is completely unacceptable, must be systematically and rigorously condemned and constitutes a security threat to us all.”
Angela Merkel and Emmanuel Macron were briefed by the Prime Minister on evidence uncovered by the British investigation, which points the finger at Vladimir Putin’s regime.
May said: “The challenge of Russia is one that will endure for years to come. As a European democracy, the UK will stand shoulder to shoulder with the EU and with Nato to face these threats together.
“United, we will succeed.”
German Chancellor Merkel told reporters in Brussels: “I have already assured Theresa May of my solidarity and our support.”
Lithuanian president Dalia Grybauskaite, hinted her country may follow the UK’s lead and expel suspected Russian spies.
Grybauskaite, whose nation borders Russia, said: “We fully support the UK in this situation and we support the measures the UK is applying, and all of us are considering such measures - the same as I am doing as president of Lithuania.”
The show of European solidarity comes after Donald Trump appeared to snub May’s plea for support.
The US President failed to raise the attack during a phone call with Vladimir Putin earlier this week and instead congratulated the Russian President on his re-election.
But EU support for May did not appear to be universal.
Jean-Claude Juncker, EU Commission President, was asked whether he regretted writing a letter to Putin congratulating him on his re-election as Russian president without mentioning the Skripal case.
He simply told journalists that Angela Merkel had done the same.
“I wrote the same letter as Mrs Merkel,” he said. “Ask Mrs Merkel.”
The EU’s high representative for foreign affairs, Federica Mogherini, stressed leaders would view the Russian threat as an entirely separate issue to Brexit.
She said: “We will obviously express our strongest possible solidarity with the UK after the attack in Salisbury, as we have done already with the foreign ministers on Monday.
″(We will be) expressing clear solidarity and deciding to stay focused on that together, because the strongest political sign we can give is unity, unity and unity - including at the moment we move forward in the negotiations on Brexit.
“This (Brexit) doesn’t mean anything in terms of diminishing our solidarity. On the contrary, we stand together.”
A Downing Street spokesman said on Thursday night: “The Prime Minister provided the President and Chancellor with a detailed update on the investigation into the reckless use of a military nerve agent, of a type produced by Russia, on the streets of Salisbury.
“She said there had been a positive identification of the chemical used as part of the Novichok group of nerve agents by our world-leading scientists at Porton Down.
“The Prime Minister also outlined our knowledge that Russia has previously produced this agent; Russia’s record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations.
“The UK, Germany and France reaffirmed that there is no plausible explanation other than that the Russian state was responsible.
“The leaders agreed on the importance of sending a strong European message in response to Russia’s actions and agreed to remain in close contact in coming days.”
No10 was also forced to underline that May had full confidence in her Foreign Secretary, Boris Johnson, after he compared Russia hosting the World Cup to Hitler’s 1936 Olympics.
Asked if Johnson’s language had made it more difficult for the Prime Minister to build a European alliance, a Downing Street spokesman said: “We will continue our work with allies on a co-ordinated response.”
Fans travelling to the World Cup were urged to follow Foreign Office advice.
“We want to ensure that our citizens are safe wherever they are in the world, as the Foreign Secretary said yesterday it’s for the Russian authorities to ensure the safety of visitors and we will be working with them on that,” the spokesman said.
Labour leader Jeremy Corbyn, meanwhile, said a “robust dialogue” was needed with Russia after he was questioned about the issue back in the UK.
“As I said, what happened in Salisbury was totally, absolutely wrong,” he said.
“But there has to be a robust, a very serious and robust dialogue with Russia.
“We live in one continent. We have to have a process where differences can be dealt with. Where we challenge human rights abuses, as I do and will and always do whoever the head of government is, whatever state it is, and we have that dialogue.
“I think at the end of the Council of Ministers summit, there will be an agreement, I hope, to condemn what happened in Salisbury. To demand the chemical weapons inspectors have access to all sites, in all parts of the world, including Russia and that we have that serious and robust dialogue with Russia.”
Corbyn said some views expressed by ministers were not “particularly helpful or sensible.”
He added: “I don’t have any problem with the people of Russia, I don’t have a problem with people of any country. Do we have a problem with people who abuse human rights? Yeh, sure we do.”