THE UK Government’s position on the ‘Russian’ nerve agent used to try to murder former spy Sergei Skripal and his daughter Yulia was beginning to unravel last night.

Following the declaration by Porton Down laboratory that it could not state for certain that the Novichok nerve agent was Russian in origin, Foreign Secretary Boris John came under fire for an earlier television interview in which he said it was “absolutely categorical” that the weapon was Russian.

Lesley Riddoch: Sudden outbreak of honesty from Porton Down belies Boris's fibs

The Foreign Office then admitted it had removed a Tweet by the UK Ambassador to Moscow Laurie Bristow in which he said: “Analysis by world-leading experts at the Defence Science and Technology laboratory at Porton Down made clear that this was a military grade nerve agent produced in Russia.”

The UK Government did win one victory, however, as the Organisation for the Prohibition of Chemical Weapons (OPCW) in the Hague yesterday voted against a proposal by Russia, China and Iran that there be joint scrutiny of the evidence.

Sergei Skripal remains critically ill in hospital in Salisbury. His daughter Yulia is said to be recovering, and the UK Government has still to decide on whether she can be seen by Russian officials, despite her being a Russian citizen.

Johnson’s problems began when Gary Aitkenhead, chief executive of Porton Down, said that although the substance used in the attack had been identified as Novichok — chemical weapons developed in Russia — it was not clear whether the material used in Salisbury had been made in Russia.

Labour leader Jeremy Corbyn, who had been slated for urging a cautious approach to dealing with Russia, said of Johnson: “He claimed categorically that it had come from Russia.

“Boris Johnson seems to have completely exceeded the information that he had been given and told the world in categorical terms what he believed had happened. And it’s not backed up by the evidence he claimed to have got from Porton Down in the first place. Boris Johnson needs to answer some questions.”

Corbyn supporter Diane Abbott said: “It seems Boris Johnson misled the public when he claimed that Porton Down officials confirmed to him that Russia was the source of the nerve agent used in the attack.”

Johnson took to Twitter to respond, writing: “It is lamentable that Jeremy Corbyn is now playing Russia’s game and trying to discredit the UK over Salisbury attack ... 28 other countries have been so convinced by UK case they have expelled Russians. In contrast, Jeremy Corbyn chooses to side with the Russian spin machine.”

Security minister and Johnson ally Ben Wallace went on the BBC to say: “Scientists are scientists. I, as well as national security, have organised crime [and] 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.

“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.”

The deleted Tweet is causing an internet furore. Ambassador Bristow’s original words as offically transcribed were: “The analysts at Porton Down, the Defence Science and Technology Laboratory in the UK, established and made clear that this was a military-grade chemical weapon. One of the novichok series; a nerve agent as I said produced in Russia

A Foreign Office spokesman said: “An HMA briefing on 22 March was tweeted in real time by @UKinRussia and amplified by @foreignoffice to explain what happened in Salisbury to as wide an audience as possible.

“One of the tweets was truncated and did not accurately report our ambassador’s words. We have removed this tweet.”

The Prime Minister’s official spokesperson said yesterday evening the Porton Down analysis was “only one part of the intelligence picture”.

The statement added the UK Government had “knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination — and as part of this program has produced and stockpiled small quantities of Novichoks.”

Why the UK Government knew about the Novichoks and didn’t do anything about it is a question the Skripals and their family and friends will surely want to ask, as will many others.