Eon will hike energy prices for over four million customers in February.
The power giant will become the fifth of the big six firms to raise costs for households this winter (see the Cheap Gas & Electricity guide).
Prices will soar by a typical 9% for electricity and 3% for gas, adding an average £58.40 to annual bills for customers who buy both fuels from Eon.
British Gas, Npower, Scottish & Southern and Scottish Power have already increased prices this winter. EDF says it won't up costs until March but it did raise prices for 1.2 million households last October.
Eon, like its rivals, blames the rising costs it pays to buy energy on the wholesale markets.
Around one million of Eon's 5.3 million customers are unaffected for now as they are either on capped or fixed rate deals; or they are on the firm's WarmAssist deal for vulnerable customers who will not be hit; or they are Age UK customers for whom the rise is delayed until April.
The increase is another reminder of the over-inflated prices paid by millions of households.
Anyone on their provider's standard tariff is almost certainly paying too much as the typical cost of such deals is around £1,200 a year whereas the cheapest online deals start at around £950.
Ann Robinson, from price comparison site uSwitch.com, says there could be a silver lining for consumers: "There is hope wholesale prices, which are the main driver behind price increases, will fall again in spring.
"If this happens consumers have every right to expect suppliers to be just as swift in cutting their prices back down again."
How to switch
To find the best tariff, compare the options available via a comparison site (see the Cheap Gas & Electricity guide).
The best deals are available to those who get their bills online and pay by monthly direct debit.
When switching, remember that when a firm describes its tariff as cheapest, it is only best value as a nationwide average.
The energy market is complicated so determining which is the cheapest provider for you depends on where you live and how much power you use.
This is why a comparison, based on your circumstances, is key.
Further reading/Key links