Tens of thousands of electricity and gas customers are facing sharply higher bills because their special tariffs are now expiring.
Some customers will see a sudden rise in their energy bills
About 80,000 Npower customers will now pay £177 a year more for their gas because their "Gas Guardian" tariff expired this week.
British Gas will raise its "Click Energy" discounted tariff for 29,000 online customers on Monday.
They will now pay 12.4% more for their gas and 9.4% more for electricity.
The Npower "Gas Guardian" tariff was introduced at a fixed price in October 2005 and was always going to expire now.
But customers who took advantage of it face a 39% rise in their gas bills, all in one go, as they were protected from the three price rises that the company brought in during 2006.
Now that they will have to pay a higher rate, their typical gas bills will rise to £628.57 a year.
A company spokesman pointed out that this sudden jump would affect only a small minority of its 6.8m customers.
British Gas had delayed imposing onto its "Click Energy" tariff customers the general increase that it brought in for all its other customers last September .
Now those who pay its online tariff will be charged, on average, £336 instead of £303 a year for electricity, while its gas customers will see their bills go up from £578 to £627.
A spokesman for the company said: "Their new tariff will still be cheaper than our standard one".
British Gas has also promised previously to cut all its tariffs later this spring, once it knows how low wholesale prices have fallen.
A number of fixed rate tariffs have been launched by energy companies in the past few years in an attempt to retain customers who have been shocked by the sharp rise in their bills.
Last year alone the average gas bill rose by 42% and that for electricity by 27%, helping to push the typical combined household energy bill to well over £1,000 a year.
The online utility switching service Simplyswitch pointed out that the customers of other major energy suppliers, such as Powergen and Scottish Power, will also be affected by similar increases.
"Many households face higher bills in the New Year as their capped and fixed-price tariffs come to an end," said Karen Darby of Simplyswitch.
"This means they will effectively be subjected to a delayed price increase."