One of Jeremy Corbyn’s most vociferous supporters – who has pushed for the deselection of MPs critical of the Labour leader – is set to face his own battle for reselection following anger at his attacks on trade unions, HuffPost UK has learned.

The Labour MP for Derby North, Chris Williamson, will “definitely” be challenged under new party rules that have made it easier to oust sitting MPs, several sources have said.

Williamson is expected to face a “trigger ballot” in his marginal Westminster constituency as local activists take advantage of changes that mean just a third of union branches are needed to force a contest.

One fellow MP said he will “get a taste of his own medicine” after spending months campaigning for some members of the Parliamentary Labour Party (PLP) to be effectively booted out.

The left-wing backbencher is popular among many members of the grassroots Momentum organisation, not least for his Democracy Roadshow campaign for automatic reselection of every Labour MP.

But what was seen as Williamson’s move to cut trade union members out of the process sparked a furious backlash, with both Unite and the GMB warning that it was an attempt to cleave Labour from its historic roots.

The issue flared up at the party’s conference last month, with some Momentum supporters shouting “shame on the unions” after they blocked more radical attempts to open up every Parliamentary seat to automatic selection.

“He’s definitely going to be triggered,” a source at one of the big trade union told HuffPost UK. “No doubt about it.”

A key figure in another major union, who preferred not to be named, confirmed Williamson would be targeted, as members wanted an MP who was less divisive and who was fully committed to the union link.

Williamson won Derby North back from the Tories in 2017, having lost the seat in 2015. He has a majority of just 2,000 votes.

Even non-union party members locally are understood to be unhappy that Williamson hasn’t been focusing enough on Derby – where Labour lost control of the council to the Conservatives this year – rather than touring the country.

“He doesn’t realise how unpopular he is,” one said. Another source said that Williamson’s decision to hold his roadshow in the seats of some strongly union-backed MPs was another factor. “If the unions get a strong enough local candidate, he could well lose.”

The party chair of neighbouring Derby South, where veteran MP Margaret Beckett is the MP, this summer tweeted disapproval of Williamson’s national roadshow.

Under Labour’s previous trigger ballot rules, more than 50% of local branches including unions were needed to spark a challenge to a sitting MP.

The new rules, which came into effect last month, mean that it takes just 33% of local union branches or member branches to force a reselection race. If there are three union branches in a constituency, only one of them is needed to set the process in train.

The party’s ruling National Executive Committee (NEC) will authorise the trigger ballot process for all local constituency parties at some point between now and the next general election.

Richard Oliver, regional political officer for the GMB, let rip at Williamson last month, declaring he and his followers were “hell bent” on “a cynical attempt to exclude Trade Union branches and their members” from local party democracy.

“The Open Selection campaign which Chris has been peddling on his Democracy Roadshow is a direct attack on the Labour Trade Union link and must be resisted.

“Whilst Derby City Council was slipping into Tory control, Chris has been turning up in Labour held seats across the country to campaign for this.”

The GMB official added the campaign was “straight from the Tory playbook” of trying to “reduce Trade Union influence by claiming to champion democracy and calling into question the Union’s democratic credentials”.

Williamson himself clashed with Unite general secretary Len McCluskey at the annual conference in Liverpool. The union boss gave the MP “a piece of his mind” over a Morning Star newspaper article that claimed Unite had abandoned its own previous support for mandatory reselection.

Trade unions are more determined than ever to put on a common front in MP selections, to allow each of them to back their own candidates in preferred seats. 

Williamson told HuffPost UK that he was confident he would win a trigger ballot if one was activated.

“It’s up to members, if they want to select someone else that’s up to them. They will make the right decision. I would hope that I would have the confidence of members and if you haven’t it’s appropriate that someone else takes over. Obviously, it wouldn’t be pleasant on an individual basis but that’s life,” he said.

“The reason I don’t like the trigger ballot system is it’s a negative process. It implies you have a problem with the sitting Member.”

“I think most trade union members would agree that greater democracy is a positive thing.”

Williamson continued: “I’m not going round slagging people off. I make a joke that there are some people I’d like to see deselected, but I always stress that’s not my decision it’s up to the local party. I think some [MPs] probably ought to be replaced, but that’s a matter for local members. 

On his Open Democracy tour, the MP said: “There’s a huge appetite for this tour. I don’t go anywhere unless I’m invited by the members. These meetings are arranged on the ground.

“I was struck by what Ed Miliband said, if we had listened to our members more we wouldn’t have gone to war in Iraq, we wouldn’t have introduced tuition fees, we wouldn’t have brought a semblance of private health in the NHS, we wouldn’t have made cuts to some of the social security benefits.

“This democracy is much wider than simply who the MP candidates are. Our membership might be electing Labour group leaders as well and I think that would be a very positive thing.”

Williamson also disagreed with criticism that he had neglected his constituency. “If people saw the caseload that my team deal with, we are dealing with thousands. I’m regularly out on the knocker talking to people on the doorstep.

“The rank and file trade union members I meet are not in my experience against more democracy.”

Williamson said that his office had been “inundated” with requests to speak in local party constituencies, and that his diary was already booked with more Democracy Tour dates up to March next year.