Unite has accused the government of acting unlawfully over its move to means-test the payment, which is expected to affect around 10 million pensioners.
The controversial decision, which Rachel Reeves said was needed to plug a £22 billion black hole in the public finances, will axe the payment of up to £300 for all pensioners except those in receipt of pension credit or other means-tested benefits.
But union Unite has called on the Department for Work and Pensions (DWP) to reverse the change, sending a pre-action protocol letter to secretary of state Liz Kendall.
It has given Ms Kendall’s department until November 7 to reverse the policy change, warning that it will ask the High Court to mount a full judicial review if not.
The Daily Express has been campaigning for the controversial cut to be dropped ever since it was unveiled by Ms Reeves in July.
General secretary Sharon Graham said: “People do not understand, I do not understand how a Labour government has taken away the fuel allowance of millions of pensioners just as winter approaches.
“Given the failure to rectify this in the budget, Unite has now commenced judicial review proceedings challenging the legality of the policy.
“It is not too late for Labour to register the hurt that this cruel policy has caused, step back from picking the pockets of pensioners and do the right thing.”
Unite is putting forward some of its members as case studies for the legal challenge, who it says are on low incomes in retirement but will no longer receive the payments.
It is claiming the government failed in its legal duty to refer the change to the Social Security Advisory Committee, which it did not.
It also alleges the government did not consider the impact on disabled people, and has said it failed to fulfil its responsibilities under the Human Rights Act.
The measure is set to save the Treasury around £1.5 billion.
It has sparked a furious backlash from the Conservatives and left-wing Labour MPs, who have branded it cruel and unnecessary.
The legal challenge comes days after it emerged ministers have been engaging in a last-minute scramble to contact 120,000 vulnerable pensioners to warn them about the policy change.
The Prime Minister previously defended the “tough” decision to cut the universal benefit.
Sir Keir said the cut was necessary because of the state of the country’s finances, accusing previous governments of avoiding confronting issues like the cost of winter fuel payments.
In Scotland, a couple has also been given permission to proceed with their own legal bid against both the UK and Scottish governments over scrapping the benefit.
