Winter fuel payment is a lump-sum amount of £200 a year for pensioners under 80, increasing to £300 for over 80s, paid in November or December.

From this winter, it will be restricted to those who qualify for pension credit and other means-tested help, meaning an estimated 10 million pensioners will no longer be eligible to receive it.

Among those are 200,000 Unite members, and the union says it is acting on behalf of retired members struggling to get by.

It says many of them have modest private pensions, making them ineligible for pension credit.

Juliet Jeater is one of 11 Unite members who have joined the union’s legal action.

She told the BBC she believed the pension credit threshold was too low, and to qualify for it “you truly have to be on the breadline”.

Ms Jeater, a retired teacher in her seventies who lives in a Northamptonshire village, said she needed the winter fuel payment to pay for heating her cottage.

In the recent cold snap, she was given scrap wood from a neighbour, who is a scaffolder, to heat her home.

A former Labour member who joined the Green Party, Ms Jeater said she was surprised to find herself better off under the last Conservative government.

She said: “I feel quite angry about what has happened.

“Last year when we had a Conservative government, I actually received £500, which was the winter fuel allowance plus a cost-of-living payment.

“This year I get nothing.”