British Gas will increase electricity prices by 12.5% from 15 September, its owner Centrica has said, in a move that will affect 3.1 million customers.
Gas prices are unchanged, but the average annual dual-fuel bill for a typical household on a standard tariff will rise by £76 to £1,120, up by 7.3%.
Centrica said the rise was a result of transmission and distribution costs and the costs of government policy.
But the government said its policy costs “could not explain” the rises.
Centrica said the price increase was its first since November 2013, adding that British Gas was one of the last suppliers to raise prices.