A Tory MP has been forced to delete a post on social media claiming Jeremy Corbyn "sold British secrets to Communist spies".
Ben Bradley, the Conservatives vice-chair for youth, removed the comment on Twitter after the Labour leader threatened legal action.
The original post, sent on Monday afternoon, said: "Corbyn sold British secrets to Communist spies… get some perspective mate!!! Your priorities are a bit awry!"
Mr Corbyn strongly denies the claim.
His spokesperson told Sky News: "Jeremy has instructed solicitors to contact Ben Bradley to delete his libellous tweet or face legal action.
"These entirely false and ridiculous smears should not be given any credence whatsoever."
It came hours before the Tory security minister Ben Wallace was accused of comparing Mr Corbyn to double agent Kim Philby who worked as a British spy before defecting to the Soviet Union during the Cold War.
He wrote: "'Jeremy has been interested in foreign policy issues his entire political career' – Labour MP Louis Haigh – yup so was Kim Philby."
A previous spokesperson for Mr Corbyn, Matt Zarb-Cousin, replied: "Ben as the UK's Minister of State for Security I'm gonna ask you to rethink this comparison between a British spy who defected to the Soviet Union and HM's Leader of the Opposition.
"Do we really want our political culture to degenerate to this level? Let's have some respect."
Mr Wallace later said he "wasn't comparing, just saying that being interested in Foreign policy isn't an answer to the allegations being made".
He told Sky News: "It was a light-hearted dig at Louise Haigh's excuse that Corbyn was interested in foreign affairs.
"I was simply saying Kim Philby was also interested in foreign affairs."
Both the Conservative Party and Mr Bradley have been approached for comment but none was received at the time of publication.
The row over Mr Corbyn's past broke out after he denied a former Czech spy's claim he was paid by the Eastern bloc to spy on Britain in the 1980s.
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Mr Bradley has previously come under pressure for a string of controversial online comments.
In January, he apologised for suggesting benefit claimants should be sterilised and penning a blog post titled: ""Public sector workers: they don't know they're born!"