The UK’s Porton Down laboratory has said it is “not our responsibility” to identify who made the Novichok nerve agent that poisoned a Russian double agent in Salisbury last month after it revealed experts could not establish the “precise source”.

In an interview with Sky News, Gary Aitkenhead, chief executive of the Defence Science and Technology Laboratory (DSTL) in Wiltshire, said it had discovered that the nerve agent used in the attack was Novichok – but was not able to prove that the poison was created in Russia.

He made clear that it was for the Government to “piece together the conclusions” through a “number of other sources”, and that it was “not our job” to say where the substance was produced.

Aitkenhead dismissed Russian claims that the nerve agent might have come from the defence facility itself, and said that its deployment was “probably only within the capability of a state actor”.

But his comments were interpreted as damning of Theresa May’s claim that it was “highly likely” that Russia is responsible for the poisoning, an argument that has rallied the West around sanctions on Moscow in response.

Others contended it showed Labour leader Jeremy Corbyn was right to call for more evidence before pinning the blame on the Russian government. 

Following the response to the interview, Porton Down, which is part of the Ministry of Defence, tweeted: “It is not, and has never been, our responsibility to confirm the source of the agent.”

Despite the clarification, the Russian Embassy seized on the interview and claimed UK Government statements on the nerve agent having been produced in Russia “were a bluff”.

“Porton Down fails to tie Salisbury poison to Russia: UK Government loses key piece of its ‘intelligence picture’ – and the rest are kept secret even from Britain’s allies,” it tweeted.

After the interview, a Government spokesman made clear there was no change in its position that Russia is to blame for the attack, arguing the Porton Down investigation is “only one part of the intelligence picture”.

Aitkenhead told Sky News: “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 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 but it’s not our job to then say where that actually was manufactured.” 

He told the broadcaster that there was no known antidote to Novichok, and that none was given to either Sergei Skirpal, 66, a former double agent, or his Yulia, 33, who is said to be in a serious but stable condition.

But in contrast to Porton Down’s remarks, some had seized on an interview Foreign Secretary Boris Johnson gave to German Deutsche Welle where he said the laboratory was “absolutely categorical” that Russia was behind the attack. Johnson said: “I asked the guy myself, I said, ‘Are you sure?’ And he said there’s no doubt.” 

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 12 March, 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.”

Aitkenhead was asked if it was possible to establish where the Novichok was made, and responded that it was working towards providing “additional information that might help us get closer”.

He said: “It can be established through a number of different input sources which the Government has access to.

“From our perspective, scientific evidence is only one of those sources and it requires a number of other things in order to verify that.

“At this stage, with the work that we’ve done thus far, we’ve been able to establish that it’s Novichok or from that family.

“We are continuing to work to help to provide additional information that might help us get closer to the question that you ask but we haven’t yet been able to do that.

“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.

“We can and we have advised the hospital and the medics on the best course of trying to mitigate against the effects but this is an extremely toxic substance and not something that you can easily give something to somebody to help them recover.”

Reacting to Aitkenhead’s comments, a Russian Embassy spokesman said: “We understood from the very start that UK Government statements on the nerve agent having been produced in Russia were a bluff.

“Now this has been confirmed by the head of the secret lab. This only proves that all political declarations on the Russian origin of the crime are nothing but assumptions not stemming from objective facts or the course of the investigation.

“This has also been essentially confirmed by today’s comment by the Foreign Office, whose ‘intelligence picture’ has lost a key piece – while the rest are kept secret even from Britain’s allies.

“We have also noted that, like in his earlier interview, Mr Aitkenhead is not denying that the lab had developed or keeps stocks of the agent they call ‘novichok’, although, of course, he would not admit it.

“By the way, some time ago we asked the Foreign Office to facilitate a meeting with Mr Aitkenhead or his colleagues, but have got no reply.

“One has to conclude that the UK Government prefers to block the Embassy from discussing the matter with experts who may possess the uncomfortable truth.”