Customers who buy their gas from Npower will see prices rise by an average of 26%, with prices for electricity customers up by 14%, both with immediate effect.
A matter of hours after that announcement Scottish Power followed suit, announcing it was increasing gas prices by an average 34% and electricity prices by 9% from Monday September 1.
The firms, which have 11.8 million customers between them, blamed soaring wholesale costs for making current pricing levels unsustainable.
They are the last of the country's suppliers to hit households with increases in recent weeks, following moves by British Gas, EDF, E.On and Scottish & Southern Energy.
Npower said the average gas customer would see an average annual increase of £162 with electricity bills rising by an average £60.
Electricity generation had also been directly affected by the soaring price of raw materials, which had increased by 122% for gas and coal and 79% for oil over the last 12 months.
Scottish Power said the average dual fuel customer would see bills increasing by 25%. About 1.2 million of the company's customers were currently protected by fixed-price tariffs, it said.
The company has offered a new tariff guaranteeing prices until December 2009.
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