Union leaders and 25 Labour MPs have urged Keir Starmer to end a “narrow, factional agenda” within the Labour party.

A letter signed by the MPs, by the leaders of several Labour-affiliated trade unions and by campaign groupings within the party, claimed the approach from the top was “increasingly unpopular with the public”.

A number of serial Labour rebel MPs are among the signatories, including Clive Lewis and Brian Leishman, as well as senior figures such as John McDonnell.

They suggested the approach by the party’s leadership could undermine the ability to win elections.

The letter comes after Starmer has faced a turbulent few weeks in his premiership, with his judgment coming into question over the decision to appoint Peter Mandelson as ambassador to Washington DC, despite his links with the child sex offender Jeffrey Epstein.

The prime minister’s top adviser Morgan McSweeney quit over his part in Lord Mandelson’s appointment, while Labour’s Scottish leader Anas Sarwar said Starmer should resign as a result of the scandal.

Further questions about the PM’s judgment were thrown up later in the week in a row over the appointment of his former spin doctor Matthew Doyle to the House of Lords after the aide campaigned for a sex offender.

Dubbed the Restore Labour Democracy statement, the letter hit out at the culture within Downing Street and Labour’s ruling team.

It said: “It’s increasingly clear that a narrow, factional agenda is being imposed upon the party and that this is increasingly unpopular with the public.

“This approach is wrong in principle and damaging in practice. When the Labour party sidelines its members, it weakens Labour’s roots in our communities, alienates voters and undermines Labour’s ability to win both people’s trust and elections.”

The signatories pointed to the candidate selection for the upcoming Gorton and Denton byelection, which Greater Manchester mayor Andy Burnham was blocked from standing in.

“Denying local members in Gorton and Denton the right to choose their candidate looks like part of a pattern, alongside the widespread barring of hard-working councillors and the withdrawal of the whip from MPs standing up for their constituents,” the letter said.

Labour MP for Leeds East, Richard Burgon, who helped to coordinate the statement, said: “Under Keir Starmer and Morgan McSweeney, there has been a clampdown on the democratic rights of Labour members.

“This has led to a small clique making out-of-touch decisions that have left the prime minister and the government deeply unpopular – from the mess over winter fuel payments to the appalling decision to make Peter Mandelson the US ambassador.

“We need an urgent end to this nasty factionalism coming from the top of the party where the views of Labour members and trade unionists are treated with contempt.

“Restoring democracy in the Labour party is essential if we are to reconnect with voters and prevent the election of a Trump-style Reform government.”

The general secretaries of Unison, Unite, the CWU, the FBU and Aslef, all Labour-affiliated unions, had signed the letter, as had the groups Campaign for Labour Party Democracy, Momentum, the Labour Muslim Network, and Labour Assembly Against Austerity.

Labour members and trade unionists will be able to add their names to the letter from 15 February.

This date has been chosen as it is the 120th anniversary of the formal adoption of the name “The Labour Party” in 1906, according to the letter’s coordinators.

A Labour party spokesperson said: “The Labour party is proud to be powered by our brilliant party members as we work to bring about the vital change we were elected to deliver.

“Our Labour government is delivering for families across the country, including tackling the cost of living, cutting NHS waiting lists and restoring pride in local communities.”