James Evans fears that pensioners, particularly in rural areas, will die of cold in the coming months because so many of them will no longer receive the winter fuel allowance.
The Brecon and Radnorshire MS said at the Senedd that pensioners are among the most vulnerable people in society.
He believes the UK Government's decision to remove winter fuel payments, worth up to £300, except for pensioners on pension credit, will place an additional and unacceptable burden on them.
Mr Evans said: “For pensioners in Brecon and Radnorshire, the removal will mean 23,898 people will lose their support. That support is not just a nice thing to have. For many, it's a matter of survival.
“Winter fuel payments have long been a vital lifeline for our older people, helping them to heat their homes during the cold winter months. Without it, pensioners face the harsh reality of rising fuel costs.
“Across the UK, energy bills have increased due to the war in Ukraine, and the cost of everything is rising.
“Many pensioners live on fixed incomes, often relying on modest pensions that barely cover the essentials.
“In Wales, nearly one in five pensioners live in poverty, with many already struggling to get the basic needs.
“They are not all well-off, as some Members think they are. Most are struggling and are living week to week.”
Mr Evans said that in rural Wales, where isolation is already a significant issue, the impact will be even more profound.
He said: “Rural fuel poverty in Wales is significantly higher than in urban areas.
“Many elderly residents live off-grid and far from support networks, and it makes it even harder for them to access support when they need it.
“Without these payments, our pensioners are at risk of fuel poverty, forced to live in freezing conditions that jeopardise their health and well-being.
“Four thousand pensioners could potentially die because of this decision, and I think most of them will be in rural areas.
“These people are our parents, our grandparents, our neighbours and our friends. We owe them more than a cold winter at home and empty promises.
“They built and fought for the communities we all call home. They deserve to live out their retirement in comfort and security, supported by a country they have supported their whole lives.
“What they don't deserve from this Labour Government in Westminster is to live in fear of hypothermia and crippling debt at home.”
Mr Evans called on the Welsh Government to take “decisive action”.
He said: “We cannot in the Senedd sit idly by while the most vulnerable members of our community in Wales are left to fend for themselves.
“The Welsh Government must step in where the Labour Government in Westminster has failed.
“Whether it's through a Welsh version of the winter fuel payments, expanding existing support schemes or working with local councils to provide direct assistance, we must quickly act to support our pensioners this winter.
“The First Minister has said on a number of occasions since taking post she will put Wales first and her party second. This is the time to walk the walk and not talk the talk.”
Senedd members then narrowly rejected Conservative calls to urge the UK Government to reverse its decision to end universal winter fuel payments.
Labour voted down the motion while Plaid Cymru joined the Conservatives in backing it, with the final tally 23 in favour and 24 against.
Leading the debate, Joel James MS warned that about 400,000 households in Wales will lose up to £300 per person after the UK Government’s decision.