The Fire Brigades Union has voted to break its historic 86-year link with Labour over the government's "betrayal" of working people.
Union bosses claim elements in the government want to smash the FBU
Delegates at the annual conference in Southport backed a resolution saying the aims and objectives of the party no longer reflected those of the FBU.
The move will deprive the party of £50,000 a year.
Relations have reached an all-time low with firefighters voting on Wednesday for a national strike ballot over pay.
'Stabbed in heart'
Delegates spoke out against New Labour over the way it treated firefighters during the long and bitter pay dispute.
Tony Maguire, from Northern Ireland, said the FBU had been "demonised" by the government during the last pay dispute.
"Our party, the party that we nurtured through the Thatcher years and the party trade unions give millions of pounds to, has stabbed us not in the back, but in the heart," he said.
"Our members have been betrayed and they feel angry and bitter and they want this mirage of a relationship with new Labour severed."
He said it was time to "start fighting like tigers" to win influence the government was denying them.
"New Labour is not listening and we have been drowned out of the big conversation."
Sandy Niven, from Strathclyde, said he had been waiting to redress the balance between the haves and the have-nots but the gap between rich and poor had continued to grow, with chief executives given rises averaging 24% last year.
"Labour is making us jump through hoops for a pay rise we are entitled to, yet still expects our members to turn the other cheek and pay money," he said.
But Mike Fordham, the union's assistant general secretary, argued that the FBU should not throw away its links with Labour because of its anger with current government policies.
He suggested slashing funding for Labour from £50,000 to £20,000 and keeping the union within the party.
His assertion that in terms of working class policies Labour was the only game in town drew shouts of "rubbish" from many delegates.
The FBU is the first union to leave Labour.
Earlier this year the Rail, Maritime and Transport Union was thrown out of the party after giving money to other political parties.
The Communication Workers' Union meanwhile has also said it will suspend its £300,000 donation to Labour if the government fails to commit to a publicly owned Post Office.
The firefighters' union has accused employers of reneging on a deal that would have seen a 3.5% pay rise backdated to November and a possible 4.2% increase from July.