UPDATE 1-RWE npower is third British energy supplier to raise prices
* Follows Centrica, SSE price hikes
* Other suppliers expected to follow suit
* Opposition leader has said he would freeze prices
LONDON, Oct 21 (Reuters) - British energy supplier RWE npower raised its household electricity and gas charges by an average of 10.4 percent on Monday, the third company to hike prices as a political row over the rising cost of living intensifies.
The leader of the opposition Labour Party, Ed Miliband, put rising energy bills at the heart of his campaign for the 2015 election last month when he said he would freeze prices for 20 months if his party wins power.
The price hike by RWE npower - almost four times the rate of inflation - will take effect on Dec.1. It is higher than rises announced by Centrica and SSE, who were the first of Britain's "big six" energy suppliers to lift their charges earlier this month.
Centrica raised its household electricity and gas by an average price of 9.2 percent while SSE announced an 8.2 percent rise.
The remaining three suppliers which make up the big six, Iberdrola's Scottish Power, EDF Energy and Eon, are expected to follow suit, with analysts saying they all face the same cost increases.
The RWE npower price hike will add 137 pounds to the average annual bill for customers taking both gas and electricity, pushing charges up to 1,459 pounds a year, the supplier said.
It was announced as Britain's energy strategy comes under the spotlight.
The government on Monday signed a 16 billion pound ($25.90 billion) deal with France's EDF to build the country's first new nuclear power station for over 20 years, agreeing to fix the price which the company will receive for the power at more than twice current market levels.
Critics say that freezing the power price for a nuclear plant for decades ahead could cause consumer bills to rise, a charge denied by the coalition government.
Npower blamed its hike on the rise in the price of the raw materials used to generate energy, the cost of fulfilling government social and environmental schemes and of using and maintaining the network which delivers power to homes, echoing the reasons cited by SSE and Centrica for their rises.
"We only aim to make around 5 pence in every pound in our retail business which we feel is a fair return for delivering reliable energy to consumers and for the risks that we bear," RWE npower Chief Executive Paul Massara said in a statement.
Energy prices in Britain have already risen by 24 percent over the last four years, according to regulator Ofgem, ramping up pressure on household finances at a time of wage stagnation, and prompting Miliband to make his price freeze pledge.
RWE npower criticised Labour's proposals. "Only 16 per cent of the bill is under our control and imposing price controls discourages investment, increases uncertainty and ultimately leads to higher prices," Massara said.
The company's electricity prices will rise by an average of 9.3 percent and its gas prices by 11.1 percent.