London (CNN) 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 last month.

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?"

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