Meanwhile one in eight said they planned to stop using parts of their homes to keep energy bills down.
More than 12,000 people over the age of 50, including almost 8000 retired people across Britain, answered the survey for Saga.
A third of people said they had been forced to stop heating as many rooms in their homes and 64% said they were having to wear extra layers of clothing as a result of rising energy costs.
One in five was planning to cut back on other spending so they could afford to heat their homes.
Just over two-fifths who responded to the online poll said they would struggle more than in the past with energy bills this winter. An overwhelming majority, 84%, expressed concern about the rising cost of fuel.
Recently the so-called Big Six firms that dominate the energy supply market all announced above-inflation price hikes.
Paul Green, director of communications at over 50s company Saga, said: "Spiralling fuel costs are striking fear in the hearts of some, but not all, pensioner households. For some, the fear could mean they won't turn their heating on, and in so doing, risk their own health and welfare.."
Jane Vass, head of public policy at charity Age UK, said: "High energy prices along with poorly insulated homes are some of the factors that cause cold homes: a major cause of excess winter deaths.
"Those living in the coldest homes are three times more likely to die a preventable death than those living in warmer ones."