Jeremy Corbyn has demanded damages from the Conservative MP who tweeted that the Labour leader had "sold British secrets to Communist spies".
Ben Bradley, a vice-chairman of the Conservative Party, deleted the tweet following a threat of libel action, but has now been issued with a letter demanding an apology and a donation to charity.
In a video posted on social media last night, Mr Corbyn said Labour would "open up" the media in response to reporting of alleged meetings with a Czech diplomat in the 1980s, who was later revealed to be a Communist agent.
Lawyers acting for Mr Corbyn wrote to Mr Bradley on Tuesday, saying his tweet was "untrue" and "could not be more seriously harmful of a British citizen, let alone such a prominent politician".
The letter states: "As the vice-chairman of the Conservative Party you are fully aware of the serious harm that was caused or was likely to be caused to our client’s reputation by your defamatory statement.
"Our client’s reputation has been or is likely to be seriously harmed by your publication of the offending tweet and by re-tweets.
"Furthermore, your tweet has been quoted in full in the Guardian newspaper, the Mirror newspaper, the Huffington Post, Sky News, the Mail Online and has been paraphrased in other national print newspapers, and online, which is unsurprising given your own high profile within the Conservative Party and your status as an MP.
"Our client instructed us yesterday evening and we advised his office to put out an immediate statement notifying you and others of the fact that he had taken legal advice and that the tweet should be deleted from your Twitter account. We note that you have removed the tweet but nevertheless serious harm has been caused by your libellous statement."
The letter goes on to demand a written promise that Mr Bradley will not to repeat the statement, a public apology on Twitter, and payment of a "substantial" sum to a charity of Mr Corbyn's choice. Mr Bradley has also been told to pay Mr Corbyn's legal costs, or face being taken to court.