ScottishPower customers will see average gas bills rise by 19% this year, while the average electricity bill will increase by 10%.
The price hike, which comes into force on 1 August, will affect 2.4 million ScottishPower bill payers, with the average dual fuel bill increasing from £1,211 to £1,391, according to comparison website uSwitch.
Customers will be notified from next week so that they have time to find a new fuel plan.
Up to 700,000 vulnerable customers will be protected from the inflated energy cost and another 700,000 customers on fixed or capped energy deals won't see any increases to their bills until their current agreements come to an end.
More rises to come
ScottishPower is the first of the big six energy suppliers to announce a second general price hike but Lisa Greenfield, energy analyst for Confused.com, expects more rises to come: "It's disheartening news for ScottishPower customers, although the other suppliers are sure to follow soon. We could be seeing a repeat of the unprecedented rises of 2008."
ScottishPower cites the increase in wholesale energy costs and unrest in oil-producing regions for pushing up its own prices.
"Wholesale prices for gas and electricity have increased significantly since the end of last year and continuing unrest in global energy markets means future prices are volatile," says Raymond Jack, UK retail director at the energy provider.
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He adds: "We understand times are difficult for many people, and we have done what we can to absorb these additional costs for as long as possible to minimise the impact on our customers."
The announcement comes just months after energy regulator Ofgem revealed evidence of providers increasing prices in line with wholesale rises faster than they decreased prices with equivalent fall in the wholesale price.
Ann Robinson, director of consumer policy at uSwitch, is calling on Ofgem to monitor these price hikes: "Ofgem will need to consider whether these increases are justified or not and act swiftly to protect consumers. Given that Ofgem has found some evidence that suppliers increase their prices more quickly than they drop them, any consumer complaints about price hikes are unlikely to fall on deaf ears."