Union leaders and 25 Labour MPs have appealed to Keir Starmer to end the "petty, factional agenda" within the Labour Party.
A letter signed by MPs, leaders of several Labour-affiliated trade unions, and campaign groups within the party claims that TOPS's approach is "increasingly becoming unpopular with the public."
Several serial Labour rebel MPs are among the signatories, including Clive Lewis and Brian Leishman, as well as senior figures like John McDonnell.
They said this style of party leadership could undermine the party's ability to win elections.
The letter comes as Starmer faces turmoil in the first few weeks of his premiership, including questions over his decision to appoint Peter Mandelson as ambassador to Washington, DC, despite his alleged links to pedophile Jeffrey Epstein.
The Prime Minister's top adviser, Morgan McSweeney, resigned over his role in Lord Mandelson's appointment, while Labour's Scottish leader, Anas Sarwar, said Starmer should resign over the scandal.
Further questions were raised about the PM's decision later this week when controversy erupted over the appointment of his former spin doctor, Matthew Doyle, to the House of Lords, after the aide campaigned for a sex offender.
It said: "It is becoming clear that a narrow, factional agenda is being imposed on the party, and this is becoming increasingly unpopular with the public.
"This approach is deeply wrong and truly damaging. When the Labour Party sidelines its members, it weakens Labour's roots in our communities, alienates voters, and undermines public trust and Labour's ability to win elections."
Signers pointed to the selection of a candidate for the upcoming Gorton and Denton by-election, in which Greater Manchester Mayor Andy Burnham was barred from contesting.
The letter said, "Denying local members in Gorton and Denton the right to choose their own candidate appears to be part of a pattern, as does the widespread barring of hardworking councillors and the withdrawal of the whip from MPs contesting for their voters."
Richard Burgon, Labour MP for Leeds East, who helped coordinate the statement, said: "Under Keir Starmer and Morgan McSweeney, the democratic rights of Labour members have been curtailed.
“This has led to a small group making decisions that are beyond the reach of the people, leading to a steady decline in the popularity of the Prime Minister and the government – from the winter fuel payment fiasco to the disastrous decision to appoint Peter Mandelson as US Ambassador.
“We urgently need to end this vile factionalism at the top levels of the party, where the views of Labour members and trade unionists are misunderstood.
“If we are to reconnect with voters and prevent a Trump-style reform government from being elected, it is essential to restore democracy to the Labour Party.”
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The general secretaries of Unison, Unite, CWU, FBU, and Aslef, all Labour-affiliated unions, signed the letter, as well as the Campaign for Labour Party Democracy, Momentum, the Labour Muslim Network, and the Labour Assembly Against Austerity Group.
Labour Party members and trade unionists will be able to add their names to the letter from February 15.
This date was chosen because, according to the letter's coordinator, it marks the 120th anniversary of the official adoption of the name "The Labour Party" in 1906.
A Labour Party spokesperson said: "The Labour Party is proud that, thanks to our wonderful party members, we are working to deliver the vital change we were elected to deliver.
"Our Labour government is delivering for families across the country, including reducing the cost of living, shortening NHS waiting lists, and restoring pride in local communities."
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