The gas and electricity supplier is expected to reveal its plans this afternoon, with estimates of the expected rises ranging from between 14 and 20 per cent in the UK's newspapers.
It is also believed that the price hikes will take the average household energy bill in the UK to over £1,000 a year.
Commenting on the reports, Adam Scorer of watchdog energywatch told the Times: "The consequence is that consumers - households, businesses, public services - all have to find extra money from tight budgets to pay for essential gas and electricity.
"Worst of all, those families who really struggle to make ends meet start to make some invidious choices about whether to spend their scarce resources on warm homes, food or clothing."
npower, which was formally established in 1999, generates and supplies energy to 6.8 million gas and electricity users across the UK.
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