Leaders of the UK's allies have backed Theresa May's claim that the suspects in the Salisbury nerve agent attack are Russian spies.
US President Donald Trump, German Chancellor Angela Merkel, French President Emmanuel Macron and Canadian PM Justin Trudeau issued a joint statement with Mrs May agreeing with the British assessment that the operation was "almost certainly approved at a senior government level" in Moscow.
The two men alleged to have been behind the March nerve agent poisoning - Alexander Petrov and Ruslan Boshirov - have been identified by the UK as members of the GRU, Russia's military intelligence service.
In the joint statement, the leaders said: "We have full confidence in the British assessment that the two suspects were officers from the Russian military intelligence service, also known as the GRU, and that this operation was almost certainly approved at a senior government level. "
They added that the announcement about the suspects "further strengthens our intent to continue to disrupt together the hostile activities of foreign intelligence networks on our territories, uphold the prohibition of chemical weapons, protect our citizens and defend ourselves from all forms of malign state activity directed against us and our societies".
Mrs May has spoken to all four of her fellow leaders in recent days to update them about the investigation.
The joint statement by leaders of "the quint" nations came as the UN Security Council - of which Russia is a permanent member - considered the Salisbury revelations.
Mrs May's official spokesman said that the Prime Minister was pleased to receive such a rapid and united response from allies.
"We got a swift response in March and we have done so again and we are thankful for that support," he said.
Former GRU officer Sergei Skripal and his daughter Yulia were left critically ill after being exposed to the military grade nerve agent Novichok in Salisbury in March.
Detectives believe it is likely the two suspects, thought to be aged around 40, travelled under aliases and that Petrov and Boshirov are not their real names.
Prosecutors deem it futile to apply to Russia for the extradition of the two men, but a European Arrest Warrant has been obtained and the authorities are also seeking the assistance of Interpol.
Mr Putin's spokesman Dmitry Peskov said that Mrs May's accusations are "unacceptable" and that "no-one in the Russian leadership" has anything to do with the poisoning, while foreign ministry spokeswoman Maria Zakharova accused the UK and US of a "witch hunt" against Russia.
The Russian embassy in the UK used its Twitter account to post a series of messages aimed at undermining the credibility of the UK investigation - including comparisons to the intelligence evidence used to build the case for the 2003 invasion of Iraq.