Energy supplier EDF has increased its electricity prices by 17% and its gas prices by 22%.
The company blamed record wholesale energy costs for the increases, which come into effect immediately.
Eva Eisenschimmel, chief operating officer of EDF Energy customers branch, said: "Record world oil prices have continued to drive up wholesale gas prices.
"Alongside unprecedented rises in wholesale coal and electricity costs, this has impacted hugely on the cost of supplying energy to our customers."
EDF Energy has announced gas and electric price hikes
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The French firm is one of Britain's biggest energy suppliers with 5.1 million customers.
The increases will mean typical customers on a dual fuel tariff will pay £3.97 a week more for their energy, it said.
The company said wholesale energy prices had increased by 70% for coal, 63% for gas and 47% for electricity since it last increased its prices in January.
Ms Eisenschimmel said: "We have been absorbing some of these costs in recent months, but we now have to pass on some of the resulting rise in wholesale costs to our customers.
"While the rise in wholesale prices is out of our control, we have been doing everything possible to keep our own costs in check.
"In this difficult economic climate we are very concerned by the impact any price rise will have, especially for those on low incomes. As a result, we have expanded the number of customers on our Energy Assist social tariff by a half, bringing the total to 100,000 customers."
SeasideMan
Pro 
Given the cost of fuels going up so much, those rises don't seem unreasonable. But, £4 a week is quite a lot to many of us. That's going to hit a lot of people pretty hard, especially when added to other rises elsewhere (food, petrol, etc.)
Tom.