Ex-deputy prime minister John Prescott has labelled unsubstantiated claims about Jeremy Corbyn’s links to “communist spies” the “dirtiest slur I’ve seen against any politician”.

Appearing on BBC Question Time, the former Labour MP called for Tory chairman Brandon Lewis to sack Ben Bradley – the Conservative vice chair who tweeted that the Labour leader had “sold British secrets” to foreign agents.

It followed a series of tabloid allegations about meetings the Labour leader and other MPs attended in the 1980s. 

Prescott said “it’s not about spies, it’s about lies”.

“Look, I’ve been 50 years in Parliament and I have to say this is the dirtiest slur I’ve seen against any individual politician.

“I’ve known Jeremy all the time he’s been in Parliament and I have to say he’s an honest man who states principles.

“It might not be agreeable to you, but he has become the elected leader of the Labour Party.

“For them now to start saying things like he is a betrayer … that in fact he now has acted as a double agent … a Kim Philby type.” 

He added how Lewis, also appearing the show, had urged Tory candidates to back a respect pledge. Reading from a pledge card, he said:

“And what it says here: ‘Conservative candidates will be suspended if they insult rivals’, the new party chairman says.

“Fine, that was the statement. I was hopeful that that might be so. But now you’ve got top people in the Tory party lined up by the papers themselves, the right-wing papers, to say that he’s a traitor, a double agent. This is unacceptable.

“Now I want to say that your vice chairman, who you appointed, made these statements about him, was selling British secrets to Communist spies.

“My God, there’s no evidence for that. Nobody’s produced any evidence. Will you now, under this new programme of better moral standards, will you now sack your vice chairman who said those things without evidence?”

Lewis responded that he was “absolutely shocked” that the Labour Party is “refusing to engage and set its own respect pledge”: “There is a very big difference between having a proper, robust debate about what people stand for, what they have said, what they have done and have been transparent about that, and personal abuse and calling for physical abuse of candidates, MPs or people in the general public.

“We are seeing (that) from the hard left, and even from the Shadow Chancellor of the Labour Party, endorsing, repeating comments like that does not fit in British politics. Labour need to call that out and deal with it.”

Corbyn has lambasted newspapers over claims he helped Communist spies in the 1980s, attacking their owners as “billionaire tax exiles” and warning them “change is coming”.

In a personal video message, the Labour leader hit out at The Sun, Daily Mail, Daily Telegraph and The Express for printing allegations that he had worked with a Czechoslovakian agent to hand over British secrets.

His angry riposte followed claims by former Czech agent Jan Sarkocy that he was on the country’s payroll during the Cold War.

Corbyn, who earlier on Tuesday made his first public comment on the claims, used the video to hit back at newspaper proprietors who “dodge paying their fair share for our vital public services”.

He said that “we’ve got news for them – change is coming”.