Labour has accused the Government of rushing to blame Vladimir Putin for the Salisbury nerve agent attack, after scientists at Porton Down said they had not verified that the substance came from Russia.
The development ratcheted up tensions between Moscow and London on the day of an extraordinary meeting of the Organisation for the Prohibition of Chemical Weapons (OPCW) in The Hague, called by Russia to discuss the case.
Mr Putin seized on the comments from the chief executive of the Government's Defence Science and Technology Laboratory (DSTL), Gary Aitkenhead, as he accused the UK of launching an "anti-Russian campaign".
Mr Aitkenhead said on Tuesday that the lab had identified the substance used to attack former Russian spy Sergei Skripal and his daughter Yulia 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 said the Government's conclusion that it was highly likely to be Russian was based on "a number of other sources".
His comments appeared to contradict the answer given by Foreign Secretary Boris Johnson when he was asked recently on German TV why the UK believed the source of the nerve agent was Russia.
"The people from Porton Down, the laboratory, they were absolutely categorical," Mr Johnson said. "I asked them that myself. I said: 'Are you sure?' He said: 'There's no doubt'."
And on March 22, the Foreign Office issued a tweet saying: "Analysis by world-leading experts at the Defence Science and Technology Laboratory at Porton Down made clear that this was a military-grade Novichok nerve agent produced in Russia."
Shadow home secretary Diane Abbott said the development raised questions about the Government's approach, and said Jeremy Corbyn should now be given credit for demanding more evidence before allocating blame.
"It doesn't surprise me Porton Down is saying this because the security services were always very cautious in what they said," Ms Abbott told BBC Radio 4's Today programme.
"What surprised me was that so many were willing to rush into the media and say it was unequivocally Putin. That's not necessarily what we were told."
Ms Abbott acknowledged that Prime Minister Theresa May had been "quite careful" in her language, but added: "Boris Johnson apparently going on international media and saying he was 101% certain it was Putin - I don't understand where he got that information from."
Mr Putin called for a thorough investigation into the poisoning during a visit to Turkey, saying that "the speed at which the anti-Russian campaign has been launched causes bewilderment".
Following Mr Aitkenhead's comments, a Government spokesman said ministers had always been clear that the identification of the substance as Novichok by the Porton Down experts was "only one part of the intelligence picture" leading to the assessment that Russian state involvement was the only plausible explanation for the attack.