Ben Bradley, a Tory party vice-chairman, tweeted the accusation, later deleting it after the Labour leader threatened him with legal action.
Now Mr Corbyn has gone further, telling Mr Bradley that “serious harm has been caused by your libellous statement” and demanding the apology, on Twitter, “in terms to be agreed with us”.
A letter sent to the Mansfield MP also says he must “agree to pay a sum of money direct to a charity of our client’s choice, in lieu of damages payable to our client for the injury you have caused to his reputation”.
It adds: “We invite your proposals by return with regard to the amount that you will pay which we would expect to be substantial, as our client’s attitude towards the level of payment will take into account the speed with which you make sensible proposals or not.”
The controversy blew up when newspaper reports said files in Czech archives appeared to show Mr Corbyn had met with a Czechoslovakian agent at least three times in 1986 and 1987.
Mr Bradley referred to the reports during an interaction with another Twitter user, saying: “Corbyn sold British secrets to communist spies.”
In the legal letter, the Labour leader states that he “does not seek any personal financial benefit from this litigation”
It says: “If you force him to issue proceedings and recover substantial damages through the courts he will donate the damages to a charity of his choice.”
Mr Bradley is also told that he must pay Mr Corbyn’s legal costs – and told that “legal proceedings” will follow, if he fails to agree to the apology and charitable donation.
“If proceedings are commenced legal costs payable by you will increase significantly, especially if the matter proceeds to a full trial,” the letter reads.
And it adds: “Any failure by you to respond swiftly will undoubtedly mean that our client’s legal costs will increase significantly.”
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