Jeremy Corbyn has threatened legal action against a Conservative MP over an online claim that he sold British secrets to "Communist spies".
A spokesman for the Labour leader said he had instructed solicitors to tell Mansfield MP Ben Bradley to take down the "libellous" tweet or face legal action.
The warning follows an exchange on Twitter between Mr Bradley and the Far Right Watch website.
A posting from Mr Bradley's account claimed that Mr Corbyn sold British secrets to Communist spies.
A spokesman for Mr Corbyn said: "Jeremy has instructed solicitors to contact Ben Bradley to delete his libellous tweet or face legal action."
There was no immediate response from Mr Bradley's office.
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".