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The precise source of the nerve agent used to poison a Russian ex-spy and his daughter has not been verified, says the head of Porton Down laboratory.
The defence research facility, which identified the substance in Salisbury as Novichok, said it was likely to have been deployed by a "state actor".
The UK said further intelligence led to its belief that Russia was responsible.
Russia's president has said he hopes a line can be drawn when the chemical weapons watchdog meets on Wednesday.
Expressing surprise at the "pace" of what he described as an "anti-Russia campaign", Vladimir Putin added that Russia wants to be part of the investigation and hopes "a line can be drawn under" the incident.
Porton Down's chief executive Gary Aitkenhead dismissed Russian claims it might have come from the UK military laboratory.
"We have not identified the precise source, but we have provided the scientific info to [the] government who have then used a number of other sources to piece together the conclusions," Mr Aitkenhead said.
"It is our job to provide the scientific evidence of what this particular nerve agent is - we identified that it is from this particular family and that it is a military grade, but it is not our job to say where it was manufactured."
A UK government spokesperson said that identifying the substance at Porton Down was "only one part of the intelligence picture".
It maintained Russia was responsible, adding there was "no other plausible explanation".
Sergei Skripal and his daughter Yulia were attacked with the nerve agent on 4 March.
The BBC understands Miss Skripal, 33, is now conscious and talking. Salisbury District Hospital has said her father, 66, remained critically ill but stable.
Mr Aitkenhead said he had been advising those treating the Skripals.
"Unfortunately this is an extremely toxic substance. There is not, as far as we know, any antidote that you can use to negate the effects of it," he added.
(BBC)
