A close aide to Jeremy Corbyn lost the endorsement of a prominent trade union in his bid for a parliamentary seat after allegations of inappropriate behaviour were made against him, the Guardian has learned.

David Prescott, son of the former deputy prime minister John Prescott, was suspended from his role in the Labour leader’s office in November after unspecified allegations were made. He returned to his role a fortnight later. It is understood he was not the subject of a formal complaint and remained a member of the party.

Prescott had hoped to win the endorsement of the influential GMB trade union to stand as the Labour candidate in Mansfield at the next election.

The seat fell to the Conservatives’ Ben Bradley in last year’s election, having been held by Labour since 1923. The nomination will be highly coveted, as Mansfield is seen as a winnable seat and has not been allocated an all-women shortlist.

The Guardian understands that Prescott initially won the GMB’s backing, which was also given to another candidate, Melanie Darrington. However, after the allegations against him emerged he was re-interviewed by the regional union branch, which decided not to proceed with the endorsement.

Nominations for the seat will close on Monday, with a longlist set to be announced next week. Prescott is still in the running and has secured two branch nominations and nominations from the Unite union and the Co-operative party.

Darrington, an aide to the former shadow cabinet minister Gloria De Piero, has officially been endorsed by the GMB. The union declined to comment.

Other candidates include the solicitor Neil Clayton and the councillor Sonya Ward, the leader of Mansfield’s Labour group. Ward has the backing of the leftwing grassroots group Momentum.

Before last year’s election, Prescott launched an unsuccessful bid to become the Labour candidate for the safe seat of Hull West and Hessle, formerly the constituency of the ex-home secretary Alan Johnson.

The Labour MP John Mann, whose Bassetlaw seat borders Mansfield, has praised the action by the GMB. Speaking at a debate in the House of Commons in December, he praised the union for building in detailed questioning of potential candidates on issues around inappropriate conduct “and for having the integrity to refuse to nominate people who do not have that understanding.”

On Thursday Mann, a vocal campaigner during the harassment scandal in Westminster, said: “We should all be proud of the GMB.”

Prescott is a key member of Corbyn’s inner circle and is credited with improving the Labour leader’s image. He has been involved in handling communications for the shadow cabinet.

Speaking to the Mansfield and Ashfield Chad newspaper earlier this month, Prescott said he would tackle the lack of regional investment as MP. “Living in north Nottinghamshire, I know all too well how towns like Mansfield have been forgotten by the Tories and are amongst the worst areas for social mobility,” he said.

Both Labour and Prescott declined to comment.