Scottish Power has been banned from using an advert promising customers that their gas bills would fall.
The advert claimed that bills were falling by 8% and that customers would save money.
British Gas complained that not every household would save money and some would see an increase in bills.
The Advertising Standards Authority agreed that not all customers would see a drop in bills and told Scottish Power not to use the advert again.
Scottish Power argued that its calculations were based on industry average consumption levels and most customers would see a drop in bills, with those who used more gas seeing a larger fall.
The company said only a small proportion of households would potentially have an increase in their bills.
It said it made this clear with a footnote on the advert.
The Advertising Standards Authority considered the claim was absolute in stating that all customers would see a drop in charges.
The authority did not agree the footnote in the ad made it clear that there could be exceptions.
It concluded that the advert was likely to be misleading.