Millions of households are bracing themselves for price hikes of 17.2% for gas and 12.7% for electricity which come into effect on Saturday.
Npower said customers will see annual bills jump on average by ?95 for gas and ?64 for electricity under the changes.
The price rises will push Npower’s average dual gas and electricity bills to more than ?1,000.
Experts have predicted that other energy suppliers will follow suit soon with prices rises of their own, spelling gloom for households this winter.
Npower – the UK’s fourth biggest energy supplier – blamed the decision to up prices on steep rises in wholesale gas and electricity costs, which it claimed have risen by more than 60% since the middle of last February.
Charities meanwhile warned that vulnerable members of society could be forced to choose between food and heating as fuel bills soar.
Old people and those who have previously been homeless will be most affected by the rise in prices, they said.
Fuel poverty charity National Energy Action (NEA) said it is “deeply concerned” about the impact of price rises on poor and vulnerable households.
Jenny Saunders, NEA chief executive, said: “These are high price rises which, for many households on low incomes, will be deeply worrying news.
“For many people, particularly the vulnerable, price hikes mean very real decisions between choosing to heat their home or doing without other essentials. Even in mild winters around 24,000 people, mostly pensioners, die of cold-related illnesses – a shameful track record in a modern country such as ours.”