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
A one tonne horse has had to be airlifted to safety by crane after getting stuck in mud in Dorney.
A driver has been arrested after being filmed spinning across a dual-carriageway before smashing into the central reservation and the verge.
A man who killed and dismembered his father told a court he gave away a Saw movie box set the day after to "disassociate" himself from them.