MI5 agents could be behind the bullying and abuse of critics of Jeremy Corbyn in a bid to stir up trouble for the Labour leader, his ally Len McCluskey, the trade union chief, has claimed.

The Unite general secretary stressed how Britain’s security services had a history of "dark practices" and suggested "right-wingers" in disguise could be responsible for the actions attributed to the Labour leader's supporters.

"Do people believe for one second that the security forces are not involved in dark practices?" he told The Guardian.

"We found out just a couple of years ago that the chair of my union then, the Transport and General Workers Union, was an MI5 informant at the time that there was a strike taking place that I personally as a worker was involved in.”

Noting how he had been on strike for six weeks in 1972, Mr McCluskey said: “Thirty years later, it comes out that the chair of my union at that time was an MI5 informant."

He went on: "Anybody who thinks that that isn't happening doesn't live in the same world that I live in. Do you think that there's not all kinds of right-wingers who are not secretly able to disguise themselves and stir up trouble? I find it amazing if people think that isn't happening."

The union chief also accused MPs and others in the party, who had complained of death threats and intimidation, of exaggerating the problem.

"There's a hysteria being whipped up,” declared Mr McCluskey. “A few people say things they shouldn't and then it's blown up out of all proportion, to suit the imagery that the Labour party has somehow become a cesspit, and suddenly it's a crisis," he added.

Earlier, Mr Corbyn was accused of allowing a "culture of bullying" to take hold within Labour and was compared to Sports Direct boss Mike Ashley by his leadership rival.

Owen Smith, the MP for Pontypridd, said "something had gone badly wrong" since the leader took charge amid fresh claims of abuse being directed at critics of the leader by activists.

Mr Corbyn insisted he was not a bully and said he was "disappointed" at the claims made against him, which included a suggestion he had threatened to ring the father of a critical MP in order to put pressure on him.

Mr Smith warned the party could be "destroyed" and "consigned to history" unless it could unite.

The former shadow work and pensions secretary told Sky News he had received death threats and the problem of abuse had not been there before Mr Corbyn's leadership win.

He accepted he had "never been bullied by Jeremy" personally but "under his leadership, there has been a culture of bullying, I fear".

He added: "Jeremy, of course, always says that he doesn't condone it but somehow under his leadership - we can't deny the facts that this wasn't something that we saw in the Labour Party before Jeremy Corbyn became leader - and it's now become commonplace in the Labour Party. So something has gone badly wrong under his watch."

Mr Smith also hit out at the suggestion Labour MPs will face having to be re-selected to stand in the 2020 general election - a move which could allow Mr Corbyn's supporters in the grassroots to oust critics.

He said: "It's not kinder and gentler, is it, if you are the boss of an organisation and the workers are unhappy, to threaten to give them the sack. It's the sort of thing you might see at Sports Direct but it's not what you should be expecting in the Labour Party."

Mr Corbyn, meantime, dismissed concerns about the prospect of re-selection hearings, insisting: "It is not Armageddon." Rather, he pointed out, it was the result of planned boundary changes as part of the UK Government's efforts to cut the number of Commons seats from 650 to 600.

The Islington MP is favourite to win the postal ballot of Labour's members - whose ranks have swelled to more than 500,000 - as well as the 183,000 people who signed up this week as registered supporters and the affiliated supporters in the unions. The result is due on September 24.