Video report by ITV News Correspondent Rupert Evelyn
Defence experts still do not know the "precise source" of the Salisbury spy attack - but are "completely confident" the nerve agent used was novichok.
Experts at Porton Down, the UK defence laboratory, are still trying to establish how the agent was administered to ex-Russian spy Sergei Skripal and his daughter Yulia on March 4.
On Monday, chief executive Gary Aitkenhead said the deployment was "probably only within the capability of a state actor".
His comments came after Russia issued a failed request to the UK to visit Ms Skripal.
Moscow's foreign minister Sergei Lavrov labelled the refusal "outrageous", adding that Britain was "putting all decency aside" as the countries expel each other's diplomats.
Mr Skripal remains critically ill in hospital, while his daughter continues to improve.
Speaking to ITV News, Mr Aitkenhead emphasised Porton Down's belief that novichok was the agent used.
But he stressed that it was the Government's conclusion that Russia was behind the attack.
Following Mr Aitkenhead's comments, a Government spokesman said: "We have been clear from the very beginning that our world-leading experts at Porton Down identified the substance used in Salisbury as a Novichok, a military grade nerve agent.
"This is only one part of the intelligence picture. As the Prime Minister has set out in a number of statements to the Commons since March 12, this includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination - and as part of this programme has produced and stockpiled small quantities of Novichoks; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets.
"It is our assessment that Russia was responsible for this brazen and reckless act and, as the international community agrees, there is no other plausible explanation."
Scotland Yard has said that it is their belief the pair first came into contact with novichok at their home.
"Our job within the whole of this investigation is and was to identify the nerve agent used - which is from the family of novichok," Mr Aitkenhead said.
"We provided that information to the Government who have then used a number of other sources to come to the conclusions that they have."
He added: "I can only emphasise that our job within this whole episode is to provide scientific facts, evidence and input - which was identification of the agent - and that is what we have done."
Mr Aitkenhead's comments also come a day before the Organization for the Prohibition of Chemical Weapons meets at The Hague to discuss the poisoning.
Theresa May announced two weeks ago the Government believed it "highly likely" that the Kremlinin itiated the attack on the double agent.
Moscow disputes the claims.
Despite the denials the UK announced it was expelling 23 Russian diplomats, with more than 25 countries following suit in a show of solidarity against Moscow.
In tit-for-tat fashion Russia declared 23 British diplomats persona non-grata, and responded to US measures by expelling 60 of their diplomats and closing a consulate in St Petersburg.