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.

Speaking to 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.

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, and all these horrible things like [the Labour MP for Wigan] Lisa Nandy’s baby being threatened are despicable.

“They’re nothing to do with me. And they’re nothing to do with what I believe in. But how can you control them? Who are they?”