Three in five elderly people will ration their heating this winter amid fears over high energy bills, according to a new survey.
As many as two in five (42%) said they would also consider cutting back on food in order to meet the cost of heating their homes, comparison website comparethemarket.com said.
The figures come as the Met Office warned of plummeting temperatures which could cause blasts of snow, frost and ice in parts of the UK, following an unseasonably warm December.
The survey of 2,000 people aged 65 and over found nearly half (46%) feared the cold snap would increase their bills, 54% worried their income or pension would not be enough to cover the cost and a fifth (22%) admitted they would have to use their savings or credit.
Some 61% admitted they cut down energy use during the winter.
Campaigners called for companies to pass on reductions in wholesale energy prices to consumers, warning that many elderly people faced a greater risk of dying from the cold.
Caroline Abrahams, charity director at Age UK, said: "The UK has an appalling record on cold-related deaths, with one older person dying every seven minutes from the winter cold. Even 'normal cold' temperatures of around six degrees significantly raise the risk of life-changing health problems such as heart attacks and strokes."
Energy and Climate Change Secretary Amber Rudd wrote to energy firms in the middle of last year asking them to pass on a fall in wholesale costs to consumers.
Wholesale prices have fallen by almost 50% over the past two years but the average household bill of more than £1,300 a year has dropped by just 14%, comparethemarket.com said.
James Padmore, head of energy insurance at comparethemarket.com, said: "It is deeply concerning that, when the temperature drops, so many of the elderly generation are rationing their fuel usage to save money, at the expense of keeping warm.
"There are other ways to reduce energy bills apart from turning off the central heating. As the wholesale cost of energy has decreased, many fixed tariffs have also fallen. However the cheapest tariffs available often require people to change energy supplier, something that the elderly in particular still seem reluctant to do."
Energy UK, which represents suppliers, said in a statement: "This winter alone suppliers will have spent in excess of £200 million supporting 1.4 million pensioners via the Warm Home Discount, a £140 rebate on their electricity bill.
"There is plenty of help available for pensioners, from discounts to grants available to help manage energy bills. It is important customers check they are on the cheapest tariff as it is possible to save hundreds of pounds by shopping around to find better deals by calling their supplier."
It urged anyone worried about their bills to call the Home Heat Helpline on 0800 33 66 99.
A Department of Energy and Climate Change spokesman said it is " taking action to support vulnerable groups who need the most help through discounts and payments in colder months".
He added: "But energy companies need to put their customers first and with wholesale costs coming down we expect savings to be passed on to them.
"The Government will keep pushing for a fairer deal by increasing competition in the market, encouraging people to switch and getting smart meters into every home by 2020."