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Gas and electricity supplier Npower is raising energy bills by an average of £64 a year for a million customers.
The average 5.3% dual fuel price hike comes into effect on 17 June and follows earlier rises announced last month by its "Big Six" rivals.
British Gas is increasing prices by 5.5% from 29 May, while Scottish Power is raising prices by 5.5% on 1 June.
German-owned Npower blamed increases wholesale energy costs and government policy changes for the rise.
Simon Stacey, managing director, domestic markets at Npower, said: "Announcing this price change today isn't a decision we've taken lightly."
He said the costs energy suppliers are facing - "particularly wholesale and policy costs which are largely outside our control" - have been on the rise for some time "and we need to reflect these in our prices".
The energy giants all partly blame government policy, such as the introduction of smart meters and emissions targets, for higher bills.
The 5.3% average price hike is made up of a 4.4% rise in gas prices and a 6.2% increase in electricity. It will see a typical dual fuel gas and electricity annual bill climb to £1,230.
Mr Stacey pointed out that the price rise would not affect existing customers on a fixed deal, those with a prepayment meter, or customers on the Safeguard tariff.
Stephen Murray, energy expert at MoneySuperMarket, said: "This is a chunky rise from Npower - all we need now is something from SSE and it's a full house from the Big Six.
"Npower says 60% of its customers won't be affected but that still means 40% - or one million people - will."
Mark Todd, co-founder of switching service Energyhelpline, said: "The most expensive standard tariff just got more expensive."
When are energy prices rising?
Some 4.1 million British Gas customers face a 5.5% hike from 29 May, adding an average of £60 to bills. The move was branded as "unjustified" by the government when it was announced.
Scottish Power is increasing prices by 5.5% - or £63 on average - for nearly one million people from 1 June.
EDF has a 2.7% - or £16 - electricity price rise coming into effect on 7 June for 1.2 million customers.
Npower's 5.3% increase - an average of £64 - will hit one million people from 17 June.
SOURCE BBC NEWS ONLINE