SCIENTISTS at the Porton Down military research facility are not able to say that the nerve agent used in the Salisbury poisoning came from Russia.

Gary Aitkenhead, Chief Executive of the UK Government's Defence Science and Technology Laboratory, said the poison had been identified as military-grade Novichok, which, he explained, could “probably” only be deployed by a nation state.

A Government spokesman was keen to stress that the authorities had always been clear that the Porton Down assessment was "only one part of the intelligence picture," suggesting there was other intelligence on which to attribute the attack to Vladimir Putin’s Government.

READ MORE: Russia calls extraordinary meeting of weapons watchdog over Salisbury attack

Indeed, Mr Aitkenhead, stressing how it was not his laboratory’s role to work out the nerve agent’s exact origin, said the Government had relied on "a number of different input sources" to come to its conclusion that it was highly likely to have come from Russia.

Nonetheless, Moscow, which has denied any involvement in the attack and has even suggested Britain was behind it, is expected to seize on the development.

Porton Down's identification of the substance used in the attack on the former double agent Sergei Skripal, 66, and his daughter Yulia, 33, 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 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 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."

READ MORE: Vladimir Putin demands thorough probe into Salisbury poisonings

The location of manufacture "can be established through a number of different input sources which the Government has access to", he explained, 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[it]; something that's probably only within the capabilities of a state actor."

Rejecting Russian claims the substance could have come from Porton Down itself, 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 Organisation for the Prohibition of Chemical Weapons[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 added.

Mr Aitkenhead’s comments came a day ahead of an extraordinary meeting in The Hague of the OPCW’s executive council to discuss the Salisbury attack.

READ MORE: Russia calls extraordinary meeting of weapons watchdog over Salisbury attack

Today’s meeting - 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.

But the Foreign Office dismissed it as a "diversionary tactic" by Russia, "intended to undermine the work of the OPCW in reaching a conclusion".

Russia has denied responsibility for the March 4 attack in Salisbury with Foreign Minister Sergey Lavrov even suggesting on Monday that it might have been carried out by the UK as a means of distracting voters from its difficulties with Brexit.

He claimed relations between Russia and the West were now worse than during the Cold War, accusing the UK and US of "putting all decency aside" in their dealings with Moscow.

Next week, the Prime Minister will travel to Denmark and Sweden for bilateral talks with her counterparts. The Russian threat will be at the top of their agenda.