E.ON has withdrawn its StayWarm tariff, which gave the over 60s fixed-price gas and electricity prices.
Customers on StayWarm deals expiring after 6 October will see their tariffs shift on to the energy company's standard E.ON EnergyPlan - which can be up to £180 more costly for some customers.
All StayWarm customers will have their deals automatically renewed to the EnergyPlan if they don't contact E.ON before their existing tariff expires.
E.ON blamed the move on its industry regulator Ofgem, which has ordered energy companies to reduce the number of tariffs they offer to four.
The reduction in the number of tariffs is seen by the government as a key step in making it easier for energy customers to get value for money by improving transparency.
Jo Ganly, energy expert at energy switching website uSwitch, said: "This is the downside to simplification, where tariffs designed for a particular group need to be withdrawn because they don't fit in to the blueprint."
In a statement, E.on said: "We're closing StayWarm to comply with Ofgem's proposed new rules. So if your StayWarm plan ends after the 6th October we'll renew you onto E.on EnergyPlan unless you call us."
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Ganly is urging customers to shop around: "E.ON will be contacting those customers affected to try to move them onto the best tariff for their needs. However, it's important that these customers also take the opportunity to shop around as there could be a far more suitable and cost-effective tariff on offer with another supplier.
"Thankfully, at the moment there are some very competitive fixed-price tariffs, which will reassure those who are concerned about rising prices. It's also a good opportunity for those who can to sign up for direct debit, dual fuel and online account management as this will help to reduce the cost of your energy bills."
She added: "If you want to shop around, but you don't have access to a computer – don't worry. You can switch over the phone with accredited comparison services like our own, or you can send a copy of your latest energy bill to 'Freepost uSwitch' with your phone number and we will run the comparison for you."