Labour MPs have demanded that candidates for the top post of party general secretary spell out their detailed plans for the movement at a hustings in parliament, amid rising fears that the dominant left is preparing a “revenge purge” of moderates.

A letter leaked to the Observer from members of the parliamentary committee of Labour MPs, the representative body of elected members, calls for the body that elects the general secretary, Labour’s national executive committee (NEC), to order the hustings so elected members can, in effect, extract guarantees from all candidates that a wide spectrum of views in the party will be respected.

The move sets the stage for what could be an electric confrontation between MPs – many of whom sought to remove Jeremy Corbyn as leader in 2016 – and the two frontrunners for the post. Both are from the Corbynite left: Jennie Formby of the Unite union and Jon Lansman, a former adviser to Tony Benn and founder of grassroots campaign group Momentum.

In a letter signed by 23 MPs, including four of the six parliamentary committee members, the group says all candidates should address and answer MPs’ questions as part of the appointment and election process. Their aim is to commit the candidates to a pluralist approach, and traditional Labour values, while ruling out purges of moderate staff and MPs, and centralised control.

In letter to the parliamentary Labour party chair, John Cryer, they write: “At the last PLP meeting it was clear that colleagues feel strongly about the importance of this post and the need for all parts of the party to feel they have a stake in the future direction of the party.

“We want to hear first-hand how the proposed holders of this crucial post will champion the diversity of traditions across the party, uphold the plurality at party headquarters and ensure we have efficient and effective operations in the run-up to the next general election.”

The position of general secretary is one of the most powerful below that of party leader. The occupant is charged with running party administration, including building membership and overseeing finances. The role fell vacant when the long-serving Iain McNicol stepped down.

Many on the left had seen McNicol as a moderate who was insufficiently supportive of the Corbyn project, and had long wanted him out. His removal has prompted new fears among moderates that the left will soon be in control of all key party positions, having already gained control of the NEC.

The vacancy has, however, opened up a power struggle between rival factions. Formby has the backing of Corbyn, while Lansman has support among many who share his vision of empowering members, and tilting the balance away from MPs.

A majority in the parliamentary party, however, fears that if either is chosen they could use the party machinery to attempt to purge moderate MPs and the many staff appointed before Corbyn became leader. The MPs will therefore seek assurances that this will not happen as well as pledges that established policymaking structures, including the party’s national policy forum, remain central. They will also want a commitment to zero tolerance of antisemitism.

Angela Smith, MP for Penistone and Stocksbridge, who is on the parliamentary committee, said: “The Labour party is a broad church and its general secretary has to respect its democratic traditions and its commitment to diversity and tolerance. Labour MPs offer a constructive role in exploring the commitment of shortlisted candidates to these critically important values and are confident that the candidates themselves would welcome such an opportunity.”

Chris Leslie, MP for Nottingham East and a former minister, who signed the letter, added: “Both of the leading candidates are from the same wing of the party – as far as I can see there’s no difference between the two of them. But given that they’re tearing each other to pieces to gain the blessing of the ‘left caucus’ on the NEC, I think it’s reasonable they also explain their different views to the NEC.”