UK Novichok allegations backed by world leaders
The US, France, Germany and Canada have agreed with the UK's assessment that Russia's government "almost certainly" approved the Salisbury poisoning.
They have urged Russia to provide full disclosure of its Novichok programme.
At a UN Security Council meeting to discuss the attack, Russia dismissed evidence presented by the UK as "lies".
The head of GCHQ has said the UK's allies would "reject the Kremlin's brazen determination to undermine the international rules-based order".
In a speech in Washington, Jeremy Fleming said: "The threat from Russia is real. It's active."
He added: "And it will be countered by a strong international partnership of allies. Able to deploy the full range of tools from across our national security apparatus."
Mr Fleming said the intelligence community had supported police in a "painstaking" and "highly complex" investigation into what happened after the poisoning of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury on 4 March.
The UK named two men, Alexander Petrov and Ruslan Boshirov, believed to be from Russia's military intelligence service, the GRU, as the main suspects.
The Kremlin called these accusations "unacceptable".
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A statement from Theresa May, Emmanuel Macron, Angela Merkel, Donald Trump and Justin Trudeau said: "We, the leaders of France, Germany, the United States, Canada and the United Kingdom, reiterate our outrage at the use of a chemical nerve agent, known as Novichok, in Salisbury on March 4."
The leaders welcomed progress in the investigation, but urged those with more information to go to UK authorities.
"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," the statement added.
Referring to the expulsion of Russian diplomats from their countries in response to the 4 March attack, the leaders said they had already "taken action together to disrupt the activities of the GRU".
The leaders said the link to the Russian service - revealed on Wednesday - would strengthen their intention to work together against foreign spy networks, the use of chemical weapons, and to "defend ourselves from all forms of malign state activity directed against us and our societies".
The UN Security Council, of which Russia is a permanent member, alongside China and UK allies the US and France, met in New York to discuss the latest progress in the investigation.
Addressing the council, Britain's ambassador to the UN Karen Pierce, said the nerve agent attack was a "direct challenge" to the "rules-based international system that has kept all of us safe since 1945".
'We have lost all hope'
In a lengthy rebuttal, Russia accused the UK of "disgusting anti-Russian hysteria".
But Russian ambassador to the UN, Vasily Nebenzya, accused the British authorities of "Russiaphobia" and misleading the international community and UK citizens.
"We have lost all hope in finding the truth," he said, adding that the latest developments were "invented out of thin air".
He highlighted a number of so-called "inconsistencies", which he said were "off the charts", in the UK's evidence.
Theresa May told the Commons on Wednesday that the two suspects had entered the UK on Russian passports.
She told MPs the poisoning was "not a rogue operation" and was "almost certainly" approved at a senior level of the Russian state.
Prosecutors in the UK believe there is sufficient evidence to charge the pair with offences including conspiracy to murder; attempted murder and the use and possession of Novichok contrary to the Chemical Weapons Act.
Russia does not extradite its nationals, but a European Arrest Warrant has been obtained in case the men travel to the EU.
Mr Skripal, 66, and his daughter Yulia, 33, were poisoned with Novichok on 4 March. Det Sgt Nick Bailey also fell ill after responding to the incident in Salisbury.
Police have also linked the attack to a separate Novichok poisoning on 30 June in nearby Amesbury, that led to the death of 44-year-old mother-of-three Dawn Sturgess and harmed her friend Charlie Rowley.