EDF Energy is to increase its standard variable dual fuel tariff prices by 6% from August, blaming a rise in wholesale prices.
The changes will affect 40% of the firm's customers, but those on a fixed price or prepayment tariff and vulnerable customers will not see any rises.
Standard variable electricity prices will go up by 6.1% and gas tariffs by 6%.
EDF said wholesale prices for dual fuel had increased by 18% so far this year.
The company announced a 1.4% dual fuel increase in April.
Managing director of customers Beatrice Bigois said: "We know that another price rise will not be welcome, and we had hoped that our limited changes announced in April would be enough.
"However, energy costs have continued to rise significantly and, despite our best efforts to absorb some of these by reducing the costs within our control, sadly we can no longer sustain this.
"Customers who wish to avoid this increase will be encouraged to choose one of our fixed price tariffs when we write to them later this month."
EDF said its standard variable dual fuel tariff will increase to £1,228 a year, a rise of £70.
The company said the 18% increase in wholesale prices since April due to a number of factors, including the Beast from the East, reducing gas storage stocks over the winter, compounded by global oil markets feeding into higher UK wholesale prices for both gas and electricity.
Kevin Pratt, consumer affairs expert at MoneySuperMarket, said: "Today's announcement has come from nowhere and really is a bolt from the blue.
"Wholesale costs have been increasing but this is the second price rise from EDF in less than three months and pushes their standard variable tariff right to the top in terms of most expensive - at £1,228, just a fraction cheaper than npower.
"The previous rise, announced in April and effective from June 7, was relatively small compared to the other Big Six suppliers, but it was still a rise and by announcing another one today they are hitting customers when they're already down."