The head of Britain's biggest trade union, Len McCluskey, has said he could envisage cutting ties with the Labour Party if it fails to win next year's general election.
Asked whether Unite could switch its financial support to another party if this happens, Mr McCluskey said: "Can I ever envisage a rule conference voting to disaffiliate from Labour? I can do."
But speaking at a Westminster lunch, the Unite Secretary General said he now "fears for the future of the party".
He warned that Labour must unveil a policy platform which shows the party is on the side of the "ordinary people," or it will suffer defeat in the 2015 vote.
Mr McCluskey said the Labour leader is now at a crossroads and must be able to provide policies to show he can offer "a genuine alternative" to austerity policies on offer from the Conservative party.
He added: "I am confident that what will emerge is a platform to take to the British electorate. I hope that it will be an alternative. If it is a pale shade of austerity then I believe Labour will be defeated at the next election.
Unite Has Donated More Than £11m To Labour Since Ed Miliband Took The Helm In 2010
"I believe that the British electorate are of a mind, unless there is a real alternative, to say: `We had better stick with the devil we know.'"
The Unite head said that his comments came at a time when Labour MPs were beginning to "grumble" over a lack of decisive Labour policies.
He said MPs are: "slightly uneasy that there isn't a cohesive vision emerging" from Labour's policy review.
There are currently reports of a battle among top party members, some of whom are pushing for a "transformative" manifesto to pull Britain away from a number of coalition policies.
Others are demanding a more cautious approach with a limited policy manifesto designed to ensure Labour secures the minimum 35% share of vote it needs to have a chance of winning power.
Britain goes to the polls in May next year.