Npower has become the latest firm to ramp up energy bills in a move that will hit nearly a million people.
The group is to increase typical annual dual fuel bills by 5.3%, or £64, across all payment types from June 17.
It is made up of an average rise of 4.4% on gas and 6.2% on electricity.
Npower blamed increases in policy and wholesale energy costs for the hike, which comes weeks after EDF and British Gas also raised prices.
Simon Stacey, managing director, domestic markets at Npower, said they had forced to pass on rising costs to consumers.
More than 60% of Npower’s customers will not face an increase, the company stressed.
Mr Stacey said: “While existing customers who are currently on a fixed deal, have a prepayment meter or on the Safeguard tariff are protected from these increases, we encourage any customer who is struggling with their energy bills to contact us.”
Energy firms across the board have been facing pressure for some time over the way they treat customers, with controversy over pricing and the use of standard variable tariffs, which have been deemed poor value for consumers.
The Government also plans to impose a price cap on costly energy products.