Defence Secretary Gavin Williamson went on the attack after reports that Mr Corbyn met an alleged Czech spy who was operating in the UK undercover as a diplomat. The Left-wing MP allegedly met one agent, who he says he believed was a diplomat, at least three times in London including twice in Parliament.
It followed an initial introduction by British members of a far-Left organisation in 1986. Czechoslovakia at the time was a puppet state of the Soviet Union and harshly repressing anti-communist dissidents.
According to Soviet-era documents from declassified secret police archives in Prague, the Czechs assessed Mr Corbyn – whom they vetted and codenamed COB in 1987 – as being “negative” towards America and “positive” towards Soviet bloc countries.
At one meeting, Czech official Lieutenant Jan Dymic was said to have raised concerns about British intelligence operations against his country’s and East Germany’s spies in the UK.
Mr Corbyn allegedly produced a copy of a newspaper article about a botched MI5 investigation and warned that British security measures might increase.
In Brussels, where he was attending Nato meetings, Mr Williamson referred to controversies about Mr Corbyn’s past contacts – which include meeting leading Irish republicans during the IRA bombing campaign and his refusal to denounce Palestinian militant violence or Iran’s hardline regime.
The Defence Secretary declared: “Jeremy Corbyn has never had Britain’s interests at heart.
“Time and time again he has sided with those who want to destroy everything that is great about this country, whether it is sympathising with terrorists, backing rogue regimes, or cosying up to those who want to inflict pain and misery on the British people.
“That he met foreign spies is a betrayal of this country. He cannot be trusted.”
Tory former foreign and defence secretary Sir Malcolm Rifkind said: “If these documents are genuine, which they appear to be, then there is a serious case for Mr Corbyn to answer.”
Conservative MP Keith Simpson, a member of Parliament’s Intelligence and Security Committee, said Mr Corbyn should have assumed that any Czech diplomat who sought a meeting was a spy.
Professor Anthony Glees, of the Oxford Intelligence Group, said: “These files show Jeremy Corbyn had been targeted by Czech intelligence services. Mr Corbyn says he didn’t know, but it shows breathtaking naivety from someone who wants to head the British Government.”
The Labour leader’s aides insisted Mr Corbyn never knowingly met a spy and had never offered confidential information to any diplomats.
A spokesman said: “The claim that he was an agent, asset or informer for any intelligence agency is entirely false and a ridiculous smear.
“Like other MPs, Jeremy has met diplomats from many countries. In the 1980s he met a Czech diplomat, who did not go by the name of Jan Dymic, for a cup of tea in the House of Commons.”
The spokesman added that spies in the Cold War were famous for telling their superiors they had “recruited” people they had only met, adding: “The existence of these bogus claims does not make them in any way true.”
Mr Corbyn risked further questions about his patriotism when he predicted Brazil or Germany would win this summer’s World Cup in Russia.
The Arsenal fan told the COPA90 soccer fan YouTube channel: “I would love to say England (will win) but I doubt it. Brazil or Germany.”