The 20,000 strong Bakers, Food and Allied Workers Union (BFAWU) will consider backing the Grassroots Out (GO) anti-EU campaign when its ruling body meets in the spring, IBTimes UK has learned.
General secretary Ronnie Draper said the union's executive will have an "open debate" on the issue and the EU referendum when it gets together in early May. But he stressed he would not share a platform with Ukip leader and GO campaigner Nigel Farage.
"We are going to watch the debate and have our own internal debate but until we have had our debate then I'm not going to say where we are – I wouldn't predict an executive decision," the union chief added.
"I have no intentions of sharing a platform with Farage or with Boris Johnson. On the other side, you've got Cameron. I wouldn't share a platform with Cameron either. All options will be discussed at our executive."
A recommendation will then be fielded to BFAWU representatives at the union's annual conference on 12 June, where members will vote on the issue with just over a week to go before the EU referendum on 23 June.
The development comes after GO announced at a 19 February rally in London that a "big" trade union would be joining the Eurosceptic umbrella group, co-founded by Labour MP Kate Hoey and Conservative MPs Peter Bone and Tom Pursglove.
But Eurosceptic rail unions the Rail, Maritime and Transport Workers and the Associated Society of Locomotive Engineers and Firemen told IBTimes UK they would not be joining GO. Left-winger George Galloway, however, has controversially sided with the group.
GO originally said it would not compete with Leave.EU and Vote Leave for the Electoral Commission's official Brexit campaign designation. But the group has since announced it plans to apply for the "leave" designation under the GO Movement banner.
The move means the group could be able to spend up to £7m ($9.8m) during the campaign's regulated period and be handed £600,000 in taxpayers' money from the Electoral Commission. The watchdog will consider, among other things, the breadth of support across the political spectrum in Vote Leave and GO.
The latest survey from Survation, of more than 1,000 people between 13 and 20 February, put "remain" on 48% and "leave" on 33%.