BIG six energy firm ScottishPower is to impose an eight per cent bills hike within weeks, it has been announced.

Anti-poverty campaigners said the change would be “a blow” for those “already struggling” to meet household heating and lighting costs.

However, the Glasgow-based firm said only two-thirds of customers would be affected, with the remaining 1.1 million exposed to the change, which was blamed on rises in wholesale energy markets and non-power schemes including de-carbonisation work and the roll out of smart meters.

Retail director Colin McNeill said: “This increase will apply to one in three of our customers, and we continue to work hard to move more customers to fixed price deals.

“We will be writing to all those affected, outlining the changes and encouraging loyal customers to move to a deal that best suits them.

“This price change follows months of cost increases that have already led to significant rises in fixed price products that now unfortunately have to be reflected in standard prices.”

From March 31, standard tariff electricity rates will go up by 10.8 per cent, with gas prices up 4.7 per cent. Typical dual fuel customers will pay an extra £86 a year, up 7.8 per cent.

The announcement came on the same day competitor British Gas said its price freeze for standard energy tariff customers will be extended until August.

It also follows an outcry when Npower revealed it is also to raise charges and advice from regulator Ofgem to providers about ways to reduce extra charges for the public.

Fraser Stewart of the Consumer Future Unit at Citizens Advice Scotland said: “Coming just a week after Npower announced a similar price rise, this news will come as an unwelcome blow for ScottishPower customers, many of whom are already struggling to pay their fuel bills.

“Energy suppliers are blaming these price rises on increases in wholesale energy prices and the cost of delivering government policies. However, only three weeks ago Ofgem told suppliers they should absorb the costs of wholesale price rises by buying energy in advance.

“We would urge all energy suppliers to be upfront and tell customers what is causing their bills to go up by such significant amounts. Consumers deserve to know exactly what they are paying for, so they are able to compare different suppliers and switch supplier if they want to.

“Anyone who is struggling with their fuel bills can get advice at their local Citizens Advice Bureau, or through our advice website.”

British Gas owner Centrica said its freeze was possible “despite increases in external costs”. Boss Mark Hodges said: “In December we promised to take action to improve how the energy market works for customers.

“We aim to do even more in the coming months to meet our customers’ needs, earn their business, and reward their loyalty.”

Npower’s 9.8 per cent hike will see its dual fuel customers pay an extra £109 a year, while rival EDF announced its rise in December.

Further rises are now expected across the market.