Ten of the UK's trade unions have slammed the conduct of the party's MPs, acknowledging Labour is heading for a leadership election.
The carefully worded statement from the leaders of unions including Unite, Unison and GMB, argue that Corbyn is the democratically-elected leader of the party, chosen under procedures backed by Labour MPs.
And while the statement stops short of directly endorsing Corbyn, it hits out at the "deeply regrettable and unnecessary" crisis engulfing the party.
"[Corbyn's] position cannot and should not be challenged except through the proper democratic procedures provided for in the party’s constitution.
"We urge all Labour MPs to abide by those procedures, and to respect the authority of the party’s leader," the unions said.
"While we have stated that we believe a leadership election would be an unwelcome distraction at this time of crisis, if one nevertheless occurs through the proper procedures we would expect all parts of the party to honour the result and pull together in the interests of the country, and working people in particular."
The party is now in its fourth day of open revolt against Corbyn. Around 60 members of Corbyn's shadow cabinet and front bench, and parliamentary aides, have resigned their roles since Sunday, with many citing the party's ineffectiveness in opposition.
Despite the furore, Corbyn yesterday vowed to battle on, despite losing a vote of no confidence 172-40 among his own MPs.
Meanwhile, Labour deputy leader Tom Watson has ruled himself out of a direct challenge on Corbyn.
Watson told the BBC earlier tonight that while he expected a leadership election, "the prerequisite for being a deputy is you never want to be leader".
The unions' letter was signed by:
Len McCluskey, general secretary, Unite the Union
Dave Prentis, general secretary, Unison
Tim Roache, general secretary, GMB
Dave Ward, general secretary, CWU
Brian Rye, acting general secretary, UCATT
Manuel Cortes, general secretary, TSSA
Mick Whelan, general secretary, ASLEF
Matt Wrack, general secretary, FBU
Ronnie Draper, general secretary, BFAWU
Chris Kitchen, general secretary, NUM