British Foreign Secretary Boris Johnson speaks during a banquet with diplomats at Mansion House in London on March 28, 2018. (Simon Dawson/Reuters)

Dozens of countries have sided with Britain against Russia after Moscow was accused of using a military-grade nerve agent to poison former Russian spy Sergei Skripal and his daughter Yulia in England last month. Boris Johnson, Britain's foreign secretary, now stands accused of publicly misrepresenting the evidence in the case.

Britain's top military laboratory said Tuesday that it cannot verify the nerve agent in the March 4 poisoning came from Russia.

Gary Aitkenhead, chief executive of the Defense Science and Technology Laboratory at Porton Down, told Sky News that although the substance used in the attack had been identified as Novichok — a class of chemical weapons developed in the former Soviet Union and Russia — it was not clear whether it had been made in Russia.

“We have not identified the precise source,” Aitkenhead said, though he added creating such a substance was “probably only in the capabilities of a state actor.”

These comments appear to contradict remarks Johnson made in an interview with German broadcaster Deutsche Welle on March 20. Asked how the British government could be so sure Russia was behind the attack, Johnson deferred to “the people from Porton Down,” who he said were “absolutely categorical.”

“I asked the guy myself, I said, 'Are you sure?' And he said there's no doubt,” Johnson said. However, it was not clear from the comments whether Johnson was saying Porton Down had confirmed the nerve agent had come from Russia or whether it was Novichok.

Diane Abbott of the opposition Labour Party called on Johnson to explain the apparent discrepancy. “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 Salisbury attack,” Abbot said in a statement Wednesday.

Labour leader Jeremy Corbyn later told reporters Johnson had “egg on his face” and that he “needs to answer some questions.”

Supporters of the government's position say it was never Porton Down's responsibility to determine the precise source of the nerve agent used against the Skripals. The British government released a statement Tuesday that said Aitkenhead's comments did not contradict the official line on the poisoning.

“This is only one part of the intelligence picture,” the statement from Prime Minister Theresa May's office said, pointing toward “our 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,” among other factors.

In a tweet on Tuesday, Porton Down reiterated this: “Our experts have precisely identified the nerve agent as a Novichok. It is not, and has never been, our responsibility to confirm the source of the agent.”

Johnson also responded Wednesday with tweets explaining his position, one of which said, “Corbyn is now playing Russia’s game and trying to discredit the UK over Salisbury attack.”

However, Johnson's comments to Deutsche Welle struck many in Britain as too ambiguous. Public opinion of the top British diplomat is mixed in Britain, in part because of his backing of Brexit ahead of Britain's vote to leave the European Union in 2016. As a journalist and frequent talk show guest before that, he was seen by many critics as using a bumbling demeanor to mask a loose grip on facts.

“Serial liar Boris Johnson caught lying again?” Kevin Maguire, an editor at the left-wing Daily Mirror, wrote on Twitter.

British journalists also noticed Wednesday the Foreign Office had deleted a March 22 tweet that said Porton Down had concluded the nerve agent used in the attack was “produced in Russia.”

The Foreign Office told the HuffPost it had deleted the tweet, which was recounting a Moscow briefing by British Ambassador Laurie Bristow, because it “was truncated and did not accurately report our Ambassador’s words.”

Russia has seized upon the discrepancies as evidence it had no role in the poisoning. In the aftermath of the attack, Moscow had offered a dizzying array of alternative theories for the poisoning, prompting Johnson to say on March 19 Russia was trying to “conceal the needle of truth in a haystack of lies and obfuscation.”

Speaking Tuesday in Ankara, Turkey, Russian President Vladimir Putin cited the Sky News interview with Aitkenhead in calling for a thorough probe into the poisoning. The Russian Embassy in Britain also noted the deleted Foreign Office tweet in a message on Wednesday.

Skripal, 66, and his 33-year-old daughter were found unconscious on a park bench in the medieval city of Salisbury last month. The elder Skripal was a former Russian military intelligence officer and double agent for Britain who had been living in Britain since 2010 after a spy swap between the two countries. He remains in serious but stable condition in a hospital; Yulia Skripal is awake and alert.

Twenty-six countries, including the United States, have joined Britain in expelling Russian diplomats after the poisoning. Moscow has responded with its own expulsions.

A number of countries have said Aitkenhead's remarks have not changed their assessment that Russia was behind the attack on the Skripals. “Nothing has changed,” German government spokeswoman Ulrike Demmer told reporters on Wednesday.

More on WorldViews

Even if a Russian hit on British soil is confirmed, what can London do about it?