TalkTalk has been instructed to withdraw a broadband advert after the Advertising Standards Authority (ASA) upheld a complaint from BT.
In the TV promotion, TalkTalk claimed that customers could save over £140 per year on internet and home phone calls by switching to its broadband services.
However, BT accused the broadband provider of exaggerating the savings that could be made by UK households.
The telecoms giant said that while some customers who had switched to TalkTalk may have saved £140, the average reduction in expenditure would be lower.
TalkTalk responded by claiming that the advert made clear this was a theoretical saving, based on the amount a customer was paying for broadband and calls before switching.
The broadband provider claimed that by including question marks on the screen when the £140 figure was quoted, it was suggesting that this level of saving was achievable, but not guaranteed.
However, the ASA took a dim view of the advert and instructed TalkTalk to avoid repeating it in its current form.
"We did not consider that the methodology was sufficiently robust to support a savings claim," the ASA ruling stated.
"The ad exaggerated the savings that customers to whom the ad was addressed were likely to achieve and was misleading."