Energy regulator Ofgem has accused npower of producing a "misleading" report about the cost of producing gas and electricity.
The firm had argued that rising costs of transporting energy meant they could do little to control the prices charged to consumers.
Npower had released documents claiming those transportation costs would increase by around 20% by 2020, but Ofgem said the data was "incorrect and misleading".
More top news
Famous faces have helped a boyfriend deliver the ultimate romantic proposal to his girlfriend of 12 years.
The Turk who shot and wounded Pope John Paul II in 1981 has laid flowers on the saint's tomb in St Peter's Basilica, the Vatican said.
Police were called after the plane was stuck at Manchester Airport for hours because of a light covering of snow, according to passengers.