Power giant E.ON today announced price rises of 9% for electricity and 3% for its gas customers from next month, blaming "rising industry costs".
The firm said the increase will add 16p a day to its dual fuel customers, adding that wholesale energy prices have soared by 35% since last Spring.
Managing director Graham Bartlett said: "It's been over two years since we last increased our prices, and, unlike other companies, we held off from increasing them during the incredibly cold temperatures we saw in late November and December.
"But we now have no option but to make this change due to increases in the prices we pay for gas and electricity, although our customers won't actually see the increases until next month."
E.ON said more than one in five of its 5.3 million customers would not be affected by today's announcement, which follows a string of price hikes by leading energy companies in recent weeks.
The latest firm to announce price increases was npower, which said last month that household gas and electricity bills will rise by an average of 5%.