Jeremy Corbyn threatened to sue a Conservative MP over an online claim he sold British secrets to ‘Communist spies’.
The Labour leader instructed solicitors to tell Mansfield MP Ben Bradley to take down the ‘libelous’ tweet or face legal action.
The warning follows an exchange on Twitter between Mr Bradley and the Far Right Watch website.
In a now deleted post, Mr Bradley – a Tory youth spokesperson – wrote: ‘Corbyn sold British secrets to communist spies… get some perspective mate!! Your priorities are a bit awry!’
The tweet was in reference to accusations that Mr Corbyn was a paid informant who would pass information on to the Czech secret police during the height of the Cold War.
Mr Corbyn strenously denied these claims at the time, calling them a ‘ridiculous smear and entirely false’.
A spokesman for Mr Corbyn said: ‘Jeremy has instructed solicitors to contact Ben Bradley to delete his libelous tweet or face legal action.’
The row comes after The Sun last week published details of documents from the archives of Czech intelligence which, it said, showed Mr Corbyn had met a Czech intelligence agent on three occasions in the 1980s during the Cold War.
The Labour leader’s office acknowledged that he had met a Czech diplomat for tea in the House of Commons but said any claim he was ‘an agent, asset or informer for any intelligence agency is entirely false and a ridiculous smear’.
A party spokesperson told Metro.co.uk on Saturday: ‘Jeremy was neither an agent, asset, informer nor collaborator with Czechoslovak intelligence.
‘The former Czechoslovak agent Jan Sarkocy’s account of his meeting with Jeremy was false 30 years ago, is false now and has no credibility whatsoever.
‘His story has more plot holes in it than a bad James Bond movie.’
Metro.co.uk has contacted Mr Bradley for comment.