BORIS Johnson has “egg on his face,” Jeremy Corbyn has insisted, as the Foreign Secretary was accused of misleading the public over the source of the nerve agent used in the Salisbury attack.

But Mr Johnson hit back, accusing the Labour leader of "playing Russia's game" and “siding with the Russian spin machine”.

As the latest diplomatic row to engulf the Foreign Secretary took off, the Foreign Office deleted a tweet blaming Moscow for the poisoning of Sergei Skripal, 66, and his daughter Yulia, 33, last month after scientists at Porton Down admitted they could not verify the source of the chemical used.

ANALYSIS: Boris Johnson appears to snatch diplomatic defeat from jaws of victory

This statement contradicted a claim made by Mr Johnson in an interview on German TV on March 20. When asked why the UK believed Russia was the source of the Novichok nerve agent, the Secretary of State replied: "The people from Porton Down, the laboratory, they were absolutely categorical. I asked them that myself. I said: 'Are you sure?' He said: 'There's no doubt.'"

A Government spokesman sought to explain Mr Johnson’s remarks, saying: "The Foreign Secretary was making clear 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," he added.

ANALYSIS: Boris Johnson appears to snatch diplomatic defeat from jaws of victory

A tweet by the Foreign Office, issued on March 22, corroborating Mr Johnson’s remarks, has now been deleted.

It said: "Analysis by world-leading experts at the Defence Science and Technology Laboratory at Porton Down made clear that this was a military-grade Novichok nerve agent produced in Russia."

A Foreign Office spokesman also sought to offer clarification on this point, saying the tweet had been an inaccurate summary of comments made by Laurie Bristow, the UK's ambassador to Russia, at a briefing in Moscow.

It was “truncated and did not accurately report our ambassador's words. We have removed this tweet," he explained.

Ben Wallace, the Security Minister, sought to defend his Government colleague, saying:"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."

But during a local election campaign visit to Watford, the Labour leader accused Mr Johnson of exaggerating the evidence.

"He claimed categorically - and he used the words 101 per cent - 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 we need a responsible, cool approach to this. We need to get to the source of this to prevent it ever happening again."

ANALYSIS: Boris Johnson appears to snatch diplomatic defeat from jaws of victory

Mr Corbyn added Mr Johnson had been left with “egg on his face”.

His Shadow Cabinet colleague Diane Abbott also challenged the Foreign Secretary to explain his earlier comments, saying it seemed he had “misled the public”.

"Boris Johnson is supposed to represent Britain on the world stage but time and again he has shown he is unable to do so responsibly," added the Shadow Home Secretary.

But the Foreign Secretary went on social media to defend himself, tweeting: “It is lamentable that Jeremy Corbyn is now playing Russia's game and trying to discredit the UK over Salisbury attack. Let's remember the key facts:

"1) Porton Down identified nerve agent as military grade Novichok; 2) Russia has investigated delivering nerve agents, likely for assassination, & as part of this programme has produced and stockpiled small quantities of Novichoks; 3) Russia has motive for targeting Sergei Skripal.”

He added: "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."

ANALYSIS: Boris Johnson appears to snatch diplomatic defeat from jaws of victory

Meanwhile, after Russia President Vladimir Putin accused the UK of mounting an “anti-Russian campaign,” Sergei Naryshkin, Director of Russia's foreign intelligence service, described Britain's claims that Moscow was to blame for the Salisbury poisoning as a “grotesque provocation rudely staged by the British and US intelligence agencies”.

In New York, Vassily Nebenzia, Russia's ambassador to the United Nations asked for a meeting of the Security Council to discuss the UK's claim Moscow was behind the nerve agent attack.