The head of the Porton Down military research facility has said his scientists have not verified that the nerve agent used in Salisbury came from Russia.
Gary Aitkenhead, the chief executive of the Government's Defence Science and Technology Laboratory, said the poison had been identified as a military-grade Novichok nerve agent which could probably be deployed only by a nation-state.
But he told Sky News it was not Porton Down's role to work out where the agent came from and suggested the Government's conclusion that it was highly likely to have come from Russia was based on "a number of other sources".
Mr Aitkenhead flatly denied Russian claims the substance could have come from Porton Down itself.
His comments come a day ahead of an extraordinary meeting in The Hague of the executive council of the Organisation for the Prohibition of Chemical Weapons to discuss the Salisbury attack.
The meeting on April 4 - to be held behind closed doors - was called by Russia to "address the situation around allegations of non-compliance" with the chemical weapons convention made by the UK against Moscow.
Russia has denied responsibility for the March 4 attack on former spy Sergei Skripal and his daughter Yulia, with foreign minister Sergey Lavrov even suggesting on Monday that it might have been carried out by the British authorities as a means of distracting voters from its difficulties with Brexit.
Mr Lavrov suggested relations between Russia and the West were worse than during the Cold War, accusing the UK and US of "putting all decency aside" in their dealings with Moscow.
Porton Down's identification of the substance used in the attack on the Skripals as Novichok was a key plank in the evidence presented by the UK in Theresa May's successful bid to recruit international support in the dispute with Moscow, resulting in the expulsion of more than 100 Russian diplomats from over 20 countries.
Asked about his scientists' findings, Mr Aitkenhead told Sky: "We in terms of our role were able to identify it as Novichok, to identify it was a military-grade nerve agent.
"We have not verified the precise source, but we have provided the scientific information to the Government, who have then used a number of other sources to piece together the conclusions that they have come to.
"It's our job to provide the scientific evidence that identifies what the particular nerve agent is. We identified that it was from this family and that it's a military-grade nerve agent. It's not our job to say where that actually was manufactured."
The location of manufacture "can be established through a number of different input sources which the Government has access to", he said, adding: "From our perspective, scientific evidence is only one of those sources, and it requires a number of other things to verify that.
"It's a military grade nerve agent which requires extremely sophisticated methods in order to create - something that's probably only within the capabilities of a state actor."
Rejecting Russian claims the substance could have come from Porton Down, Mr Aitkenhead said: "There's no way that anything like that would ever have come from us or leave the four walls of our facilities.
"We've got the highest levels of security and controls. We are regularly audited by the OPCW to make sure we are operating within those controls. If there was any hint that anything that we have would be leaving our four walls, then we wouldn't be allowed to operate."
He described the nerve agent as "an extremely toxic substance" with no known antidote.
Porton Down was able to advise doctors on the best treatment to mitigate its effects on the Skripals and Wiltshire policeman Nick Bailey, who was affected after coming to their aid, he said.
Meanwhile, retired Russian Lieutenant-General Evgeny Buzhinsky warned that relations between Russia and the West could become "worse" than the Cold War and "end up in a very, very bad outcome" following the nerve agent attack.
Asked to spell out what this would mean, he told BBC Radio 4's Today programme: "A real war, worse than a cold war is a real war, it will be the last war in the history of mankind."
A Downing Street spokesman said: "As the Prime Minister has made clear, the UK would much rather have in Russia a constructive partner ready to play by the rules.
"But this attack in Salisbury was part of a pattern of increasingly aggressive Russian behaviour, as well as a new and dangerous phase in Russian activity within the continent and beyond.
"As the Prime Minister has said, we must face the facts, and the challenge of Russia is one that will endure for years to come."