Scottish Power is raising its gas and electricity prices by 5.5% for nearly one million of its customers.
The move, which will come into effect on 1 June, will see typical standard dual-fuel bills for direct debt customers rise by £63 a year, to £1,211.
Those who pay quarterly will see an average increase of £85.
Scottish Power said the price change would apply to about one third of its customers, or 960,000 homes.
Neil Clitheroe, chief executive of Scottish Power's retail division, said the increase in standard variable prices reflected "rising wholesale energy costs and compulsory non-energy costs".
He added: "We will be contacting all customers affected by the price change to give them the opportunity to move to a fixed-price tariff alternative and avoid this increase."
Scottish Power is the third major energy firm to announce price increases this year.
Earlier this month, EDF Energy said it was raising its electricity prices by 2.7%, days after British Gas raised prices for 4.1 million of its customers.