Tens of thousands of pensioners will face a hike in energy prices after E.ON withdrew its discount tariff for over-60s.
The StayWarm deal, which offered fixed-price energy for older customers based on their average consumption levels, will come to an end at the weekend.
E.ON says the move has been forced by changes to the number of tariffs permitted by regulator Ofgem. But it will leave older StayWarm customers searching for new deals when the plug is pulled on the tariff on Sunday.
A company spokesman said: "Due to new Ofgem rules, which includes limiting the number of products we can offer, the StayWarm tariff will close as current contracts come to an end from October 7.
"We are writing to all customers as their contracts come to an end, and where possible we are also contacting them by phone with the aim of speaking to each customer to help ensure they switch to the best product for their needs."
StayWarm, which allowed older people to plan their energy bills throughout the year, had been available to households inhabited by at least one person aged over 60.
The premium paid depended on the number of inhabitants and bedrooms, as well as geographical region.
But it had been unavailable for new customers since September last year.
The Government's energy regulator Ofgem is currently reforming the market in order to simplify the bills, although consumer watchdog Which? has criticised its plans, claiming companies will still be allowed to include a standing charge as well as a unit price in their tariffs.
The company described the array of charges as ''bewildering'' and the market ''too confusing for consumers to find the best deal for them''.
An investigation two weeks ago - focused on the available range of gas, electricity and dual fuel deals and standing charges available to a specific customer in one region of the country - showed there were more than 109 tariffs that included 75 different standing charges.
It found these standing charges varied from zero up to £402 a year on individual gas and electricity deals combined and by as much as £373 on a dual fuel tariff.
Which? has called for simple tariffs, without standing charges and displayed in the style of petrol forecourt prices, to make it easier to spot the cheapest deal.