Video report by ITV News Correspondent Rupert Evelyn
Boris Johnson is at the centre of a row over the production of the nerve agent used in the Salisbury poisoning.
Labour leader Jeremy Corbyn said the Foreign Secretary has serious questions to answer after the Porton Down laboratory said it had not identified the source of the Novichok nerve agent used in the March 4 attack.
This was despite Mr Johnson appearing to claim the laboratory had stated it was produced in Russia – a claim later repeated in a now deleted tweet by the Foreign Office.
The Foreign Secretary accused Mr Corbyn of “siding with the Russian spin machine” after the Labour leader said there had been a “huge inconsistency” in the information being put out.
In a series of tweets, Mr Johnson said it was “lamentable” that the Labour leader was “playing Russia’s game”.
On Tuesday, Porton Down chief executive said the laboratory's role had not been to establish where the nerve agent had come from - adding that the Government's conclusion was based on "a number of other sources".
Mr Corbyn said the Foreign Office had caused confusion with their statement.
"There clearly was a huge inconsistency in that the Foreign Secretary had made a statement, the Foreign Office put out a tweet in support of what he’d said," Mr Corbyn said.
"Porton Down then said they couldn’t and wouldn’t identify where it had come from, so they then deleted the tweet.
"Where does that leave the Foreign Secretary? Egg on his face for the statement he made on German television."
When challenged on German TV about his claim that Russia was the source of Novichok, Mr Johnson said Porton Down were “absolutely categorical”.
ITV News Political Correspondent Paul Brand reported the Foreign Office clarification after the Russian Embassy brought focus to the tweet on March 22.
Analysing the spat, Brand said the government "overstretched" in "claiming Porton Down had made the link for them".
Former Foreign Secretary, Sir Malcolm Rifkind told ITV News: “You never in any criminal investigation get 100% evidence. Juries are instructed, they can convict people of murder if they are satisfied with the evidence beyond reasonable doubt and that is exactly what we have here. Beyond reasonable doubt – a Russian nerve agent was used to kill a Russian who had been a double agent.
“Putin has said on television in the past that these people will never be forgiven and it appears there is intelligence available to our intelligence agencies shared with other governments that the Kremlin for a number of years have had a programme of taking action against individual Russians whom they believe is hostile to their states’ interests.”
The development comes as an extraordinary meeting of the Organisation of the Prohibition of Chemical Weapons (OPCW) convened in The Hague on Wednesday.
Russia complained about what they perceive as "non-compliance" by the UK government, including their decision to refuse the Russian embassy access to Ms Skripal in hospital.
At the meeting, the British delegation described Moscow's calls for joint UK/Russian investigation as "perverse".
The Foreign Office meanwhile has accused the Kremlin of attempting to undermine the work of the OPCW and engaging in "another diversionary tactic".
An FO spokesman said: "Russia has called this meeting to undermine the work of the OPCW, which, fully in accordance with the chemical weapons convention, is providing the UK with technical assistance and evaluation through independent analysis of samples from the Salisbury attack.
"Of course, there is no requirement in the chemical weapons convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator.
"This Russian initiative is yet again another diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion."