Energy giant RWE npower heaped more pressure on households today by hiking electricity and gas prices by 9.3% and 11.1% respectively from December 1.
The 10% average bill increase will affect about 3.1 million customers, the group said, and follows recent price hikes by British Gas and SSE.
The announcement came as Britain sealed a deal with French energy EDF to build the first nuclear power station in a generation at Hinkley Point in Somerset, after lengthy wrangling over taxpayer subsidies.
Npower blamed the higher cost of delivering power to homes, meeting Government schemes and rising fuel costs, insisting it makes a "fair return for delivering reliable energy to consumers".
The price hike will mean an average dual fuel customer's bill rises by £137 a year, from £1,323 to £1,459.
Npower, the third of the so-called "Big Six" to drive through bill increases, also raised tariffs by 8.9% last November.
Mark Todd, director of price comparison service energyhelpline, said: " This is devastating news and is proof to cash-strapped consumers that customer loyalty reaps no benefits.
"The truth is that there is nowhere to hide from the energy hikes and more suppliers are set to follow suit."
Bill hikes will add more upward pressure to stubbornly high inflation, which currently stands as 2.7%, far outstripping average wage growth of just 0.7%.
Npower's announcement follows Labour leader Ed Miliband's pledge to freeze energy prices for 20 months if his party wins the next election, and is likely to intensify the debate over the soaring cost of living.
But RWE npower chief executive Paul Massara said a bill freeze would not lead to lower sustainable prices because it would not cut the cost of supplying energy, adding that the German group only aims to make a profit of around 5p in the pound.
He said: "Only 16% of the bill is under our control and imposing price controls discourages investment, increases uncertainty and ultimately leads to higher prices."
Npower insisted more than 500,000 customers on fixed contracts will not be affected.
British Gas recently announced it is hiking electricity bills by 10.4% and gas tariffs by 8.4% - affecting 7.8 million households - while SSE said it is hitting seven million customers with an 8.2% rise.
Customer-owned energy firm Co-operative Energy also recently announced a tariff hike of 4.5%, and called on its larger rivals to put customers before profits, saying it was absorbing some of the increasing costs of buying wholesale energy.