Almost one million Scottish Power customers will see their gas and electricity bill rise by 5.5 per cent.
The energy giant announced new increases affecting 960,000 people - a third of all customers - with an average hike of £63 a year for those with the standard dual-fuel package.
Those who pay quarterly will see an even greater rise of approximately £85.
The new pricing plan will come into force from 1 June.
Neil Clitheroe, chief executive of Scottish Power’s retail section defended the surge and stressed most customers would be unaffected.
He said: “Unfortunately our standard variable prices are increasing. This reflects rising wholesale energy costs and compulsory non-energy costs.
“Two-thirds of our customers are unaffected. 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.”
The increase is well ahead of the current UK inflation rate of 2.8 per cent, which the Bank of England said Wednesday was at its lowest level in a year.
Consumer groups were quick to criticise the jump in price.
Victoria Arrington, spokesperson for price comparison and switching service energyhelpline said: “The energy price rise wave is full on at this point – the majority of the Big Six have now announced price rises happening within the coming weeks.
“Scottish Power’s rise is only the latest of the stinging hikes eating into the pay cheques of people across the UK, and this is the biggest price rise so far from a major supplier – a real whopper.
She added: “No energy customer should be on a standard tariff as they are almost always bad value for money.”