Jeremy Corbyn said he has been subjected to "lies and smears" over accusations that he knowingly passed information to a Communist spy during the Cold War.
The Labour leader said newspapers which made the reports were going "a little bit James Bond" over the "increasingly wild and entirely false" claims.
He lashed out at rightwing newspaper owners, who he said were running scared of a potential Labour Government, in a video statement posted on social media.
The video was filmed and released by Mr Corbyn's office. He refused requests from ITV News to answer questions about the matter.
- Jeremy Corbyn declines to answer ITV News questions
The allegations are based on Mr Corbyn's contacts with former Czech spy Jan Sarkocy during the Cold War in the 1980s.
Mr Sarkocy today told ITV News that Mr Corbyn knew that he was a member of the Soviet country's secret police during their meetings. He also claimed the politician was paid for information which he supplied.
Those allegations were strongly disputed by Svetlana Ptacnikova, the director of the Czech Secret Services archives, who said she did not believe Mr Sarkocy was a credible source.
She said the security services’ archives prove that Mr Sarkocy did meet Mr Corbyn several times, but there wasn’t a shred of evidence to suggest Mr Corbyn was paid for information or knew that Mr Sarkocy was a spy.
Mr Corbyn has already flatly denied that he ever knowingly met a spy or passed on sensitive information.
He has threatened Tory MP Ben Bradley with legal action over a tweet claiming he "sold British secrets to Communist spies".
Mr Corbyn reacted dismissively when questioned over the claims by a Daily Mail reporter at a press event.
In his latest statement, Mr Corbyn said a free press was "essential for democracy" but added: "At the moment, much of our press isn't very free at all."
"In fact it's controlled by billionaire tax exiles, who are determined to dodge paying their fair share for our vital public services.
"The general election showed the media barons are losing their influence and social media means their bad old habits are becoming less and less relevant.
"But instead of learning these lessons they're continuing to resort to lies and smears."
The Labour leader's office acknowledged that Mr Corbyn had had tea in the Commons with a Czech diplomat, but said any claim he was "an agent, asset or informer for any intelligence agency is entirely false and a ridiculous smear".
A spokesman for The Sun stood by their story and said it was backed up by "substantial, documented evidence from the Czech Security Archive".
A statement from the newspaper said the reporting of the story was in the public interest and Mr Corbyn should answer questions over his meetings with Mr Sarkocy.
"Those questions are yet to be answered and we will keep asking them, no matter how inconvenient they might be, nor how many times we are threatened with 'change' - whatever that may mean.
"They are questions that we would put to anybody who aspires to the highest office in the land.
It added: "Mr Corbyn has been admirably open and transparent on issues such as his tax return - a story we have reported on more than once, as well as his demand for the Prime Minister to do the same.
"We look forward to such openness on this issue, too."