Jeremy Corbyn has threatened a Conservative MP with legal action over claims he sold British secrets to "Communist spies".
Mansfield MP Ben Bradley was warned to take down the "libellous" tweet which made the claim about the Labour leader.
It comes after The Sun ran an article last week which contained details of documents from the archives of Czech intelligence which, it claimed, showed Mr Corbyn had met a Czech intelligence agent on three occasions in the 1980s during the Cold War.
The latest row followed a Twitter exchange 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."
After The Sun published its article, the Labour leader's office acknowledged that he had met a Czech diplomat for tea in the House of Commons.
But it said any claim he was "an agent, asset or informer for any intelligence agency is entirely false and a ridiculous smear".