Scottish and Southern Energy (SSE) is set to increase gas and electricity prices for its customers from September.
An 18% average rise in gas prices and an 11% increase in electricity prices, due to be implemented from 14 September, will mean higher fuel bills for millions.
The company cited increased wholesale energy prices as one of the reasons for the decision. SSE said wholesale costs of gas have gone up by 40% and that of electricity by 23% since December when it last raised prices.
The company, which owns Southern Electric, Swalec and Scottish Hydro, added that it has also been facing higher costs of distribution networks usage and making investments into social and environmental initiatives.
But it added that there would be no further increases until 1 August next year.
Scottish and Southern Energy is the third major supplier after Scottish Power and British Gas to push up prices.