First Utility is set to scrap its iSave V12 tariff, leaving customers with 19% more to pay each month, on average.
Over a year, this amounts to an average of around Â£200.
Customers on the iSave V12 tariff will automatically switch to the iSave Everyday tarrif on June 1.
Although the 19% price rise is unusually high, the iSave Everyday tariff is covered by First Utility's price promise, which, it says, guarantees cheaper energy prices than the standard tariffs of British Gas, ScottishPower, EDF, Eon, SSE and npower - the 'big six' energy suppliers.
First Utility's founder and chief financial office Darren Braham said: "Although First Utility has been subject to the same challenging cost increases as other energy suppliers, we chose to delay changing our prices for as long as possible, and until customers were through the coldest winter months.
"The big six all announced price rises by the end of 2012 during a period when heating your home costs you the most."
A First Utility spokesperson said that the price hike was part of a simplification of the company's tariff structure, and assured customers they will not face a penalty for choosing to change tariffs, or switch energy suppliers.
First Utility is one of the most popular energy providers outside of the big six.
Last year, it launched a pilot scheme in which customers could sign up for energy efficiency SMS alerts. These warned customers if they were using higher-than-normal amounts of energy, and reminded them to take energy efficiency measures, like turning down thermostats on boilers.
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