Britain's claim that Moscow was behind the poisoning of a former Russian spy faced fresh scrutiny Wednesday when it emerged the UK government had deleted a tweet blaming Russia for producing the nerve agent used in the attack.
The UK Foreign Office acknowledged it had removed the social media post, which stated that British chemical weapons experts believed Russia made the nerve agent, Novichok, used to poison Sergei Skripal and his daughter Yulia in the English city of Salisbury on March 4.
A Foreign Office spokesperson told CNN that the tweet was removed because it was "truncated and did not accurately report" a briefing by the British ambassador to Moscow on March 22.
The revelation came as the UK government was already on the defensive on the issue. On Tuesday, the chief executive of the Porton Down defense laboratory in the UK said scientists could not confirm where the nerve agent was made. The British government insisted that additional material from intelligence sources led it to the conclusion that Russia was behind the attack.
Russia, which has repeatedly denied involvement in the attack and has sought to blame Britain instead, seized on the developments. The Russian embassy in London tweeted: "Why would @foreignoffice delete this tweet from 22 March?"
The controversy centered on a briefing on the Salisbury poisoning given by the British ambassador to Moscow, Laurie Bristow. In it, he said "the analysts at Porton Down, the Defense 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 tweet, which the Foreign Office said was published in "real time", paraphrased the comments, saying: "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."
The tweet was deleted sometime before March 27. An identical tweet by the UK embassy in Russia, was also deleted.
The Foreign Office spokesperson said the decision to delete the tweets did not change the UK government's assessment that Russia was responsible for the "brazen and reckless act" of poisoning the Skripals.
Britain and its allies, including the US, France and Germany, have expelled more than 100 Russian diplomats from their countries over the poisoning, prompting a series of reciprocal expulsions of foreign diplomats from Russia.
Porton Down contradicts Johnson
News of the deleted tweets came a day after Gary Aitkenhead, the head of the Porton Down laboratory said that while scientists were sure that only a "state actor" could have accessed the nerve agent, they could not say definitively where it was made.
"We have not identified the precise source, but we have provided the scientific info to government, who have then used a number of other sources to piece together the conclusions you have come to," Aitkenhead told Sky News.
He added: "It is our job to provide the scientific evidence of what this particular nerve agent is. We identified that it is from this particular family and that it is a military grade, but it is not our job to say where it was manufactured."
His comments contradicted remarks made by Foreign Secretary Boris Johnson, who was asked last month how the government was able to quickly conclude that Russia was the source of the nerve agent.
"When I look at the evidence, the people from Porton Down, the laboratory, they were absolutely categorical," Johnson told the German broadcaster Deutsche Welle.
"I asked the guy myself, I said: 'Are you sure?' And he said: 'There's no doubt.' So we have very little alternative but to take the action that we have taken," said Johnson.
UK chemical weapons representative slams Russia
The brewing controversy over the tweets threatened to overshadow an emergency summit of the world's chemical weapons watchdog in The Hague on Wednesday to discuss the poisoning of the Skripals.
The British delegation to the Organization for the Prohibition of Chemical Weapons (OPCW) slammed a Russian proposal for a joint investigation into the incident as "perverse," describing it as a "diversionary tactic" aimed at evading questions over Moscow's alleged role in the attack.
Speaking at the meeting, called at Russia's request, the UK said the Kremlin's response to the Skripal case, as well as chemical weapons attacks allegedly committed by the Syrian regime, were proof of its disdain for international monitors.
"It seems clear that Russia will never accept the legitimacy of any investigation into chemical weapons use unless it comes up with an answer Russia likes," said John Foggo, the UK representative to the OPCW.
The European Union's representative at the chemical weapons meeting, Krassimir Kostov, also backed the UK's assessment that it was "highly likely" Russia was behind the attack, telling delegates of the EU's "shock at the offensive use of any military-grade nerve agent, of a type developed by Russia, for the first time in European soil in over 70 years."