SHADOW home secretary Diane Abbott, asked if she was surprised Porton Down had potentially suggested the poison might not be from Russia, told BBC Radio 4's Today programme: "It doesn't surprise me Porton Down is saying this because the security services were always very cautious in what they said.

"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."

Told Prime Minister Theresa May did not say this, Abbott replied: "She didn't say that, she was quite careful in her initial statement. But 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."

Abbott added she hoped Labour will "get some credit for taking a more thoughtful approach and asking the right questions".

The director of Russia's foreign intelligence service Sergei Naryshkin described Britain's claims that Moscow is to blame for the Salisbury poisoning as a "provocation".

Russian news agency Tass reported Naryshkin telling the Seventh Moscow Conference on Global Security: "Even as far as the Skripal case goes, which is a grotesque provocation rudely staged by the British and US intelligence agencies, some European countries are in no hurry to follow London and Washington, preferring to sort the situation out."

Britain's delegation to the Organisation for Prohibition of Chemical Weapons has branded Moscow's call for a joint UK/Russian investigation into the Salisbury poisoning "perverse".

In a tweet issued as the OPCW's executive council met at Russia's request in The Hague, the UK delegation said: "Russia's proposal for a joint, UK/Russian investigation into the Salisbury incident is perverse.

"It is a diversionary tactic, and yet more disinformation designed to evade the questions the Russian authorities must answer."

In a statement on Boris Johnson's comments in a March 20 interview on German TV, a Government spokesman said: "The Foreign Secretary was making clear that Porton Down were sure it was a Novichok - a point they have reinforced.

"He goes on in the same interview to make clear why, based on that information, additional intelligence and the lack of alternative explanation from the Russians, we have reached the conclusion we have.

"What the Foreign Secretary said then, and what Porton Down have said recently, is fully consistent with what we have said throughout. It is Russia that is putting forward multiple versions of events and obfuscating the truth."

Labour leader Jeremy Corbyn has accused Foreign Secretary Boris Johnson of indulging in "a bit of exaggeration" over the source of the Novichok nerve agent.

Corbyn, while on a visit to Watford, said: "He claimed categorically and I think he used the words 101% that it had come from Russia. Porton Down have not said that, they said that they've identified it as Novichok, they cannot identify the source of it.

"Either the Foreign Secretary has information that he's not sharing with Porton Down or it was a bit of exaggeration. I don't know which it is, but I think we need a responsible, cool approach to this.

"We need to get to the source of this to prevent it ever happening again."

The UK's permanent representative to the OPCW John Foggo has accused Russia of showing "disdain" for the organisation's independence.

Foggo told an extraordinary meeting of the body's executive council in The Hague: "Russia's refusal to accept the results of the OPCW's investigation unless Russian experts participate in it suggests that Russia is opposed to the independence and impartiality of the Technical Secretariat and is nervous about what the results will show.

"Russia's statements demonstrate a wilful ignorance of the Convention's provisions and, worse than that, a disdain for the independence and competence of the Technical Secretariat."

In a statement to the OPCW on behalf of the European Union, Bulgarian ambassador Krassimir Kostov, said: "We have full confidence in the UK investigation and laud UK's collaboration with the OPCW Technical Secretariat, in full compliance with the convention."

Kostov said the EU supports the UK's demand for Russia urgently to address questions about the Salisbury attack.

And he added: "Today we confirm once again this position and reiterate our solidarity with the people and the Government of the United Kingdom and those fighting for their lives after the attack."

Liberal Democrat leader Sir Vince Cable said the findings from Porton Down had not changed his mind over what happened in Salisbury.

"It hasn't changed my view. I was a Cabinet minister for five years, I worked very closely with the intelligence agencies," he told the Press Association.

"I trust them. I think they're highly professional and objective, and what they tell us and what the Prime Minister has said is that the overwhelming circumstantial evidence is that the Russians were involved, and I believe that.

"I've many other differences with the Government, but I do support their position on this issue."

The European Commission said it had never thought that Porton Down was tasked with identifying the source of the substance used in Salisbury.

Commission spokesman Alexander Winterstein told a Brussels press briefing: "Our understanding is that the role of the experts there was to identify the type of agent that was used, not the source of the agent.

"That's also what the experts have done. They did identify the nerve agent Novichok. That is what they have done."

Security minister Ben Wallace said the Government took time to assess the evidence and intelligence before accusing Russia.

He told BBC Radio 4's The World At One: "Scientists are scientists. I, as well as national security, have organised crime, terrorism under my portfolio, and when we work with forensic scientists, the scientists tell us what something is - they tell me a gun and a type of gun was used - but the attribution of who used it, exactly how it was used is a matter for the broader investigation.

"That includes intelligence, detectives if it's a police investigation, and the scientists as well, and that's perfectly understandable."

Questioned why Foreign Secretary Boris Johnson appeared to suggest he had received assurances from Porton Down that Russia was the source of the Novichok, Wallace said the Russians have "stockpiled" the nerve agent and have "used chemicals and other things".

Pressed further, Wallace said: "Porton Down will be able to tell you there are very, very, very few people in the world who, first of all, did design Novichok - and that was the Russians - and who have developed and stockpiled it.

"In fact the task of that is reduced to one."

Russia's Ministry of Foreign Affairs has said that a relative of the victims of the Salisbury attack, Viktoria Skripal, is planning to visit them in hospital.

"Russian Embassy (in the UK) contacted with Viktoria Skripal, relative of the Russian victims of the Salisbury incident, Sergei and Yulia Skripal," said spokeswoman Maria Zakharova in a series of tweets issued by the MFA.

"Our diplomats share with her the latest information available to the Russian side about the medical condition of her relatives.

"Viktoria Skripal plans to visit her relatives in the UK to provide moral and psychological support. We consider this an absolutely natural and sincere desire, and especially important now, as Yulia Skripal's condition has reportedly improved."

Zakharova added: "We are ready for a comprehensive, highly professional and open co-operation on the Skripal case with the UK both in a bilateral format and within the OPCW, including with the participation of OPCW Technical Secretariat and other interested parties."