Five of the UK's Big Six energy suppliers have increased their prices following Ofgem's move to raise the energy price cap.
Scottish Power today became the fifth of the major players to announce price increases, coming on the back of British Gas's decision this morning and Npower, Eon and EDF last week.
The move leaves SSE as the only Big Six firm not to hike prices.
Switching service Energyhelpline estimates that 8.9m people will be affected by the latest price increases, even more if SSE joins its peers.
The suppliers have all matched Ofgem's new price cap, increasing bills by almost £120 to around £1,254 for the average energy user on a default tariff.
Dermot Nolan, the chief executive of Ofgem, said: "We can assure these customers that they remain protected from being overcharged for their energy and that these increases are only due to actual rises in energy costs, rather than excess charges from supplier profiteering."
The government regulator set the price cap as a measure to protect consumers from rip-off energy deals. The cap gets reviewed twice a year, with changes implemented in April and October.
Peter Earl, head of energy at Comparethemarket, said consumers who expected the cap to protect them will "feel sadly let down".
"Our data shows that over the last month, the Big Six lost more customers than it gained, as households voted with their feet and sought out more competitive deals. Even with the price cap in place, 90 per cent of current fixed tariffs deals available on the market are cheaper than the price cap level," he added.
Scottish Power's 10 per cent hike will affect 900,000 costumers, adding a combined £105m to annual bills around the UK.
British Gas's increase was the largest, at 10.5 per cent, which will cost 3.5m customers £409m more per year.
With prices set to increase across the five companies, Richard Neudegg, head of regulation at uSwitch, urged consumers to consider alternatives.
“There are nearly 200 energy tariffs which are currently cheaper than the new price cap will cost. Consumers can grab savings of over £300 by switching now,” he said.