Eight Labour-backing trade unions have urged the party to end "internal wrangling" caused by ex-leader Jeremy Corbyn's ongoing suspension as an MP.
They say the situation is creating "deep division" and Labour will not be "forgiven" by voters if it continues.
The Community union, which backed current leader Sir Keir Starmer in the contest to succeed Mr Corbyn, is among those asking for a "quick" resolution.
Mr Corbyn was punished over remarks on the scale of anti-Semitism in Labour.
Following the publication of a damning report by the Equalities and Human Rights Commission in October, he said opponents and the media had "overstated" the size of the problem.
Labour suspended his party membership but he was readmitted last month, after saying he regretted any "pain" caused.
However, Sir Keir decided not to restore the party whip to Mr Corbyn, meaning he currently sits as an independent MP.
Labour Chief Whip Nick Brown has said the former leader must "unequivocally, unambiguously and without reservation apologise" for his comments.
The unions' statement says that, on 23 November, leaders met Sir Keir to express "concerns" and ask him to reinstate Mr Corbyn as a Labour MP.
It adds: "In the face of millions losing their jobs and a disgraceful imposition by the government of a public sector pay freeze, we have been distracted by internal wrangling, which is causing deep division in our party and will not be understood or forgiven by the British public.
"We urge all parties to resolve this matter quickly."
A previous statement objecting to Mr Corbyn's suspension from Labour in October was signed by the leaders of seven of the 12 Labour-affiliated unions: the Communication Workers Union (CWU); the Fire Brigades Union; the Aslef and TSSA transport unions; the BFAWU bakers' union; the National Union of Mineworkers; and the general union Unite.
These are generally seen to be on the left of the Labour movement, as is Mr Corbyn.
Community, which represents textile and metal workers and is considered more centrist, told the BBC there would be no further comment on why it was backing the latest statement, not having supported the previous one.
Labour has been contacted for a response.
In an interview with GQ magazine, former Home Secretary Alan Johnson, who led the CWU before becoming a Labour MP, said the relationship between unions and the party was "vital".
He added: "Sometimes the leaders of these unions - voted in on very small turnouts - have their own political notions and that's when you get a problem."
General secretaries should "represent all their members, including those who don't support Labour", Mr Johnson said.