Leaders of the Fire Brigades Union will today decide whether to re-affiliate to Labour, more than a decade after it left the party.
The union, which has around 38,000 members, split from Labour in 2004 following a bitter pay dispute with the then government of Tony Blair.
General secretary Matt Wrack said there had been a "remarkable" turn of events since Jeremy Corbyn was elected as Labour leader earlier this year.
The FBU was one of the first trade unions to come out in open support of Mr Corbyn.
Mr Wrack said: "We have seen a remarkable turn of events during the summer, which we hope will change the political landscape to make for a fairer, more just Britain.
"The Labour leader is fully and unreservedly pro trade unions. Jeremy Corbyn and John McDonnell (shadow chancellor) have supported the FBU for many years.
"In fact, more than a decade ago they co-founded the FBU's parliamentary group, of which they are still both members."
A special conference is being held in Blackpool to decide on re-affiliation.