Jeremy Corbyn met with a Communist spy three times during the height of the Cold War, it was alleged.
Politics professor Anthony Glees, who directs the Centre for Security and Intelligence Studies at The University of Buckingham, told Mike Graham and Katie Perrior: "Nobody ever thought he would be leader of the Labour Party, least of all Jeremy Corbyn, and he thought that gave him a licence [to] cosy up to people who are the enemies of freedom".
He was said to have met with Soviet-backed agent Lieutenant Jan Dymic twice in the House of Commons and once elsewhere. A spokesman for Mr Corbyn said that he had met a diplomat, but never knowingly talked to a spy.
"The existence of these bogus claims does not make them in any way true", he stressed.
Some of the information supposedly held in the secret files is not exactly earth-shattering.
For instance, it quotes the files as describing Corbyn as "negative towards US, as well as the current politics of the Conservative government".
Corbyn's antipathy towards the United Kingdom and USA governments at a time when Margaret Thatcher and Ronald Reagan were their respective leaders will surprise no-one.
The Sun front page is the latest in a series of allegations thrown at Corbyn since he became Labour leader, which intensified during last year's general election campaign.
Corbyn really has them rattled!
The global development secretary, Penny Mordaunt, is to meet the boss of the National Crime Agency (NCA) to discuss the Oxfam aid worker sex scandal.