Energy supplier E.On is to increase its standard variable dual fuel prices by an average of 8.8% from April 26.
The energy giant said its standard electricity prices will increase by an average of 13.8% and standard gas prices will rise by an average of 3.8% "due in large part to increasing policy costs and other costs it doesn't control".
Around 2.5 million of E.On's residential customers - 62% - will "potentially" see an increase if they take no action before April 26, the company said.
E.On's announcement follows a spate of energy price rises in recent months.
Npower, EDF and Scottish Power are among those to have ramped up bills, with many blaming rising wholesale costs.
Co-operative Energy is also increasing the cost of its standard variable tariff by an average of 5%.