The TV, press and internet ads stated that existing customers could fix their energy prices for no extra cost, with small print which read "conditions and exclusions apply."
A number of customers complained that the ads did not make it clear that some existing customers would have to switch to a higher tariff to qualify for the deal.
The ASA ruling said: "We considered that the presentation of all the ads implied the offer was available to all existing customers and the disclaimers therefore contradicted the main claim in the ad rather than clarifying it."
Clearcast said it did not believe the TV ad stated or implied that the offer was available to all existing customers.
However, the ASA said the ads must not be shown again in their current form.
.jpg)
.jpg)