Labour has been accused of playing “student union politics” after blocking a candidate from standing in the Nottinghamshire seat of Bassetlaw because of allegations about her behaviour.

Sally Gimson had been selected by local members in the constituency to stand for the seat vacated by John Mann.

But she received a letter last week saying that a series of complaints had been made about her conduct, including at meetings in her current local constituency party of Holborn and St Pancras. She says she was not given full details.

After a hearing on Monday with a national executive committee (NEC) panel chaired by the Momentum chair, Jon Lansman, and conducted by telephone, Gimson was told on Wednesday that the NEC would not be endorsing her candidacy.

She was told the complaints related to behaviour in respect of a person with a protected characteristic. These include age, disability, gender reassignment, race, religion or belief, sex, sexual orientation, marriage and civil partnership and pregnancy and maternity.

Labour sources suggested some of these complaints predated her selection, though she was still shortlisted by the NEC.

“I have been thrown out on trumped-up charges that only came to light five days ago,” Gimson said. “From the outset I was not the preferred candidate – a Londoner from Keir Starmer’s constituency and a firm remainer. The NEC had already worked to keep Bassetlaw applicants off the longlist and I got through the process with the belief that the favoured candidate would annihilate me.

“Their calculations were wrong and my strong campaign, speaking and meeting with local members, meant that I won the selection.”

She retains the support of the Bassetlaw constituency Labour party, which was holding a meeting on Wednesday evening to consider how to proceed.

Jo White, the CLP’s chair, complained that the process had been opaque and last-minute. She said: “I joined the Labour party to campaign for a Labour government and Labour policies and to make a difference to society – and part of my commitment to that is transparency and openness and decency, and what they’ve done to her is completely the opposite of that.” White said local members were “incandescent”.

Gimson is likely to be replaced as the candidate in Bassetlaw by Keir Morrison, a councillor in nearby Ashfield who was widely believed to be the NEC’s favoured candidate in the first place.

Lord Mann, the former MP, said: “The people in Bassetlaw will not accept or vote for a Momentum stooge. People liked Sally Gimson because she was a strong personality.

“Momentum had stitched up the selection already to exclude good local candidates and now want to change the result because their candidate was not chosen. Local people won’t have it. It is crude – the worst kind of student union politics.”

Labour members in Bassetlaw chose Gimson from a three-person shortlist little more than a week ago, and had already produced campaign literature bearing her name.

Holborn and St Pancras CLP released a statement supporting her. “At no point was the CLP contacted by the NEC regarding these complaints and there has never been any prima facie evidence provided to the CLP that would warrant any formal investigation into Sally Gimson’s conduct at any of our meetings,” it said.