Scottish Power has announced plans to increase its gas and electricity bills within a few weeks.
Gas bills could leap by an average of 17 per cent, while electricity bills could rise by ten per cent.
Scottish Power, which has 5.2 million customers, said wholesale energy costs were to blame for the hikes.
The firm said wholesale costs had surged by 80 per cent over the last 12 months.
Scottish Power, which last month revealed a 47 per cent leap in annual profits to £675 million, said around one million customers would avoid the rises because they are on a capped price deal.
Willie MacDiarmid, ScottishPower's director of energy retail, said: "Wholesale energy costs continue to rise and are now a record 80 per cent higher than this time last year.
"We have absorbed most of the impact and our last price increase announced in February was one of the smallest in the sector. However we now need to pass on some of these increased wholesale energy costs".