Midlands-based npower yesterday signalled winter misery for households after it confirmed it would hike standard gas and electricity prices.
The UK's fourth biggest energy supplier said it would provide more details on the tariff rises today.
However, a spokesman said the percentage increases would be in the double digits for both electricity and gas.
The move marks the first by an energy supplier to increase standard rates after last spring's price cuts.
Npower's hike is likely to be the first of a flurry of similar increases across the sector as suppliers seek to pass on rises in the cost of wholesale energy prices.
Npower, which is based in Worcester and owned by German company RWE, has already increased its online tariff for new customers, upping bills by 17 per cent the week before Christmas.
British Gas parent Centrica also last month announced increases to its market tracker price, increasing bills by 13 per cent for gas and 15 per cent for electricity.
It also warned less than two weeks later that wholesale prices left the energy industry facing a "difficult environment" in 2008, signalling that prices were set to rise across the board.
Npower's announcement today came as no surprise to industry experts, who saw the group's online price hike as a precursor to a wider tariff review.
Tim Wolfenden, head of home services at uSwitch.com, said: "Ever since British Gas and npower announced increases on their market trackers and npower followed this with a hike on its online plan, it's been clear prices are going to be heading north again, with the smart money on a 15 per cent rise.
He warned that npower's decision to up standard rates will "give competitors the green light" to follow suit.
Wholesale gas prices have been driven higher by soaring oil costs, with crude touching $100 a barrel for the first time late on Wednesday.
It is thought that wholesale prices in the UK have also been driven higher as European firms increasingly turn to the more liberalised UK market for supplies as a cheaper alternative.
Data for gas flows in the sub-sea pipeline that provides a link between UK and Continental European energy markets show gas was largely flowing from the UK to Europe during most of December.
The energy markets across Europe are said to be less competitive than in the UK, while European wholesale energy prices also tend to be higher, as they are more closely linked to the cost of oil.
A drop in wholesale energy prices in 2006/early 2007 enabled suppliers to cut bills, with British Gas the first to announce decreases. Npower dropped its prices at the end of April 2007.
