Pensioners could be losing up to £50,000 in benefits over their lifetime, according to new research from British Gas and Help the Aged.
One in three pensioners said they did not know where to get advice
Half of those entitled to more help could claim enough to cover all their fuel costs.
The charity says benefits must be paid automatically if the government is to meet its fuel poverty target.
Pensions Minister Mike O'Brien insists "every effort" is being made to ensure pensioners claim the correct benefits.
The research was carried out by the London School of Economics (LSE) for the British Gas Help the Aged Partnership's winter deaths campaign.
Figures from the Office for National Statistics show that 22,300 people over 65 died in England and Wales last winter.
Help the Aged identifies fuel poverty as a major cause of many of those deaths.
"Recent research by the British Gas Help the Aged Partnership revealed that over one million older people cut back on food to cover their heating costs," said spokesman Anna Pearson.
"If the government is serious about meeting its fuel poverty commitment it must stop gambling with the lives of older people in winter, and recognise its current strategy for delivering benefits is completely off the mark," she added.
In 2001 the government pledged to lift all vulnerable households out of fuel poverty by 2010, but many campaigners fear it will not meet its target.
The LSE report found that around half of pensioners who were entitled to additional help could receive enough money to cover all their heating costs, lifting them out of fuel poverty entirely.
Analysing the typical amount of unclaimed benefit, it suggests a sixty-five year old woman who claimed an extra £25 a week would receive more than £30,000 over the rest of her expected lifetime.
Claiming an extra £50 a week would be worth £50,000.
But the report found one in three pensioners did not know where to turn for help with benefits.
Help the Aged wants the government to introduce the automatic payment of benefits and invest in more face to face advice.
Pensions Minister Mike O'Brien said "The government is making every effort to ensure that pensioners take up the benefits they are entitled to.
"This year around 11.6 million pensioners in more than 8 million households will receive Winter Fuel Payments of £200, rising to £300 for people over 80.
"We're spending £800 million on improving heating and insulation in low income households through initiatives such as Warm Front," he added.
He said the government was committed to "moving towards" more automatic payments of benefits.