Intelligence services posing as Jeremy Corbyn supporters could be behind the abuse and intimidation of MPs on social media in an attempt to “stir up trouble” for the Labour leader, the Unite boss Len McCluskey has suggested.

In an interview with the Guardian, the general secretary of the UK’s largest trade union and one of Corbyn’s strongest supporters said he thought “dark practices” would ultimately be uncovered by the 30-year rule, under which classified documents are released into the public domain three decades after being written.

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Asked if he believed the online abuse of Corbyn’s critics was posted by people trying to discredit his supporters, McCluskey said: “Of course, of course. Do people believe for one second that the security forces are not involved in dark practices?

“I have been around long enough … the type of stuff that we ultimately find out about, about who was involved in who, the 30-year rule.

“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. [In] 1972, I was on strike for six weeks. And 30 years later it comes out that the chair of my union at that time was an MI5 informant.”

Asked again if he believed that classified documents would eventually reveal the involvement of security forces in Corbyn’s leadership difficulties, McCluskey said: “Well I tell you what, 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 rightwingers who are not secretly able to disguise themselves and stir up trouble? I find it amazing if people think that isn’t happening.”

McCluskey said he believed that MPs and others who had spoken of death threats and intimidation were exaggerating the extent of those threats.

“There’s a hysteria being whipped up,” he said. “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.”

McCluskey said he condemned the threats, but he was unsure what politicians could do to stop it. “God knows how you can control social media,” he said. “I know the more vicious elements hide their identities... But how can you control them? Who are they?”

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A Whitehall source with knowledge of the intelligence services rejected the accusation. “MI5 are focused on protecting the country and its people from the very real threat of terrorist attacks and would never - and could never - engage in this type of activity,” the source said. “MI5’s activities are subject to rigorous oversight and to the law.”

Facebook Twitter Pinterest Angela Eagle, Jeremy Corbyn and Tom Watson. Relations at the highest level of Labour’s parliamentary party have been strained in recent weeks. Photograph: Carl Court/Getty Images

Reacting to McCluskey’s claims, Angela Eagle, the Labour MP who challenged Corbyn for the leadership before dropping out to endorse fellow rival Owen Smith, said: “I’ve known Len for 40 years. I worked with him and other trade unions on combating the Tories’ awful trade union bill and on stopping Sunday trading changes, and he knows I spent years working for the trade union movement. His comments are over the top.”

The day after Eagle announced she was to challenge Corbyn, a brick was thrown through a window of the building where her constituency office is based. Her local constituency party, which has threatened to deselect Eagle, was suspended this week amid allegations of bullying.

Last week a man in Paisley, Renfrewshire, was arrested on suspicion of threatening to kill Eagle. He has since been released on police bail.

“These are serious issues,” Eagle said. “Rape threats, death threats and organised bullying are not something to be ignored or minimised. We have a democracy and we need Labour politics of solidarity to avoid the kind of anger and hostility that the politics of division inspires.

“We need to unite the Labour movement so we can take on the Tories and heal the country, not continue to ignore the cancer of political violence that has been unleashed in the last few months.”

Former home secretary Jacqui Smith called McCluskey’s comments “a downright insult to the dedicated staff of MI5 who are actually working day and night to protect us from terrorism.”

Jacqui Smith (@Jacqui_Smith1) A downright insult to the dedicated staff of MI5 who are actually working day & night to protect us from terrorism https://t.co/GI7RZEqQeQ

McCluskey’s relationship with Labour MPs who back Smith for the leadership has significantly deteriorated in recent weeks, including with his former flatmate Tom Watson.

The Labour deputy leader had tried to get the union boss to be the mediator to heal the divisions between the parliamentary party and trade unions and the leadership, but McCluskey later alleged that Watson had sabotaged the process by pulling out of the last round of talks, which had been ongoing intermittently for several days.Watson said talks had ended when Corbyn had made it clear his leadership was not up for negotiation under any circumstances.

On a blog for the Huffington Post, McCluskey said Watson had “no scruples” and had underestimated the unity and strength of union backing for Corbyn.

McCluskey said he had initially not wanted to support mandatory reselection of Labour MPs, but said the rebellion against Corbyn had made him unwilling to argue against it. A motion calling for Labour to adopt it was submitted to Unite’s policy conference last week.

“My executive had decided to oppose it,” he said. “And had the executive opposed it, I think the conference would have opposed it. But the mood changed dramatically, and the executive supported it.

“This crazy vote of no confidence [in Corbyn], this mass resignation, this coup attempt – and you know, it’s Newton’s third law of motion, isn’t it? For every action there is a reaction. And it changed me. So although I asked the executive not to take that position, I didn’t go into the ditch to die for it. I voted with the executive.”