ScottishPower is raising the cost of its standard variable gas and electricity tariff by 3.7%, piling further pressure on squeezed households.
The energy firm said the changes will come into force on October 8 and hammer 900,000 customers with an average £46 added to annual bills.
It blamed “continued increases in wholesale energy costs”.
Neil Clitheroe, chief executive of ScottishPower’s retail arm, said: “We have seen significant increases in wholesale energy costs since April, and, like others in the industry, this means that we need to increase our prices.
“More than two-thirds of our customers are on fixed-price products or other tariffs not impacted by this price change.
“Those customers affected by the price change will be contacted and offered the opportunity to move to a fixed-price tariff alternative and avoid this increase.”
It is the second time this year that the Big Six energy company, which is owned by Spanish multinational Iberdrola, has increased prices.
The firm increased its standard variable gas and electricity prices for around a third of its customers from June 1, with 950,000 households hit by an average increase of 5.5%.
The latest hike comes just weeks after British Gas said it is raising its standard variable tariff by 3.8%.
Energy regulator Ofgem has also recently increased its price cap on variable tariffs due to rising wholesale prices.
Its safeguard tariff, which protects five million households from overcharging, will go up by £47 per year in October to £1,136.
The watchdog said it was increasing the cap due to rising oil prices, which have fed through to wholesale gas prices.
Several energy companies have ramped up prices over the summer, affecting millions of households.
E.ON, SSE, Npower, EDF and Bulb have all hiked bills, blaming wholesale energy costs for the increases.
Consumer groups have warned energy customers about the rising tariffs, saying they should switch to a better deal.
Victoria Arrington, a spokeswoman for energyhelpline, said: “Consumers can hardly keep up with the tsunami of price hikes this year. This second ScottishPower price rise is yet another bitter pill to swallow for millions of households.”