The Unite union has called on the government to reverse its decision to cut the winter fuel payment for millions of pensioners or has warned it may face judicial review.

The government announced plans to cut the payment for most pensioners, worth between £100 to £300, in July, a move confirmed in the Autumn Budget.

Labour has faced criticism over the decision, which Chancellor Rachel Reeves said she took in order to help plug a £22 billion hole in public finances.

Unite general secretary Sharon Graham said in a statement that “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”.

An estimated 10 million pensioners will lose their winter fuel payment. The benefit will still be paid to those who receive pension credit or other means-tested help.

A pre-action letter from Unite’s legal representation, sent to the government on 29 October, argued that the government failed to undertake a full assessment of how the decision would impact people.

The government revealed in September that there had been no wide-ranging assessment, but a more limited “equalities analysis” was released.

The letter named Works and Pensions Secretary Liz Kendall as a proposed defendant. The government technically has 14 days – until 12 November – to respond, although the union requested a response by 7 November.

The government has been asked for comment.

The letter also said the government had a legal duty to refer the cut to the Social Security Advisory Committee and should have gathered more evidence about the impact, particularly on vulnerable and disabled people, by law.

The matter was “urgent” due to increasingly cold weather and the significant impact on people “who are at risk of disconnection, and who are increasingly having to forgo adequate heating and cut other essential spending”, the letter read.

“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,” Graham added.

Prime Minister Keir Starmer 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.