Scottish Power, the UK's fifth biggest energy supplier, has raised the price of its gas and electricity.
From 1 March, the firm's gas prices will rise by 15% and electricity by 8%.
The firm blamed a record 80% rise in wholesale energy costs in the past year for its decision to increase prices for its 5.2 million customers.
It has been widely reported that British Gas, the UK's biggest energy company, is set to raise its prices by up to 25% soon.
Scottish Power increased its gas and electricity prices last October.
The firm's director Willie MacDiarmid said the company was continuing to absorb much of the impact of increasing wholesale costs.
He said: "We have worked hard to protect our customers from this over the winter. However, we now need to pass on a further part of the increased costs from March when milder weather should help lessen the impact on bills."
'Winter protection'
Consumer watchdog Energywatch Scotland called on Scottish Power to take immediate action to ensure its most vulnerable customers could afford to stay warm and keep their lights on.
Energywatch spokesman Graham Kerr said: "Customers can escape the rise by switching to a cheaper supplier. There is simply no reason to continue paying too much for gas and electricity."
He added: "The timing of the announcement, a day after reporting healthy profits, will jar with many customers and make them question just why they should be getting such bad news the day after Scottish Power itself had such good news."
Scottish National Party energy spokesman, Richard Lochhead MSP, said the rises would force more Scots into fuel poverty.
"It is a scandal that in energy-rich Scotland, fuel poverty is escalating," he said.
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