The group, which has 5.2 million customers, said electricity prices will rise by an average of 14%, with gas prices up by 15%.

The move follows double-digit price hikes last month from rivals British Gas, Npower and EDF.

Scottish Power said customers with dual-fuel tariffs faced an average increase of 14.8%. The new prices will not impact 1.2 million customers on capped or fixed price deals.

The Glasgow-based firm, which is owned by Spanish energy firm Iberdrola, said the increases were necessary after the cost of coal rose by 97% and wholesale gas prices lifted by 83% on February last year.

Scottish Power said its bills were still cheaper for combined gas and electricity than those offered by British and Scottish Gas.

Director of energy retail Willie MacDiarmid said: "We have worked hard to protect our customers from these increases for as long as possible and offer consistently competitive energy prices."

Scottish & Southern Energy has pledged to leave prices unchanged until the end of next month, while German-owned E.On is expected to make an announcement soon.

Scottish Power's electricity price rises will vary by region, but a spokesman said most customers would see a 14% increase. That is equivalent to a £45 increase to an annual £367 for customers paying monthly by direct debit.

The 15% rise in gas equates to a £79 increase to £604, while the typical dual fuel bill will increase by £123 to £959.