The Advertising Standards Authority (ASA) has rapped BSkyB on the knuckles for an ad campaign "implicitly attacked BT's business practices".
The ads watchdog upheld a complaint from BT, which accused BSkyB of running a "misleading and denigratory" ad campaign.
The Sky Talk adverts, which ran last year, stated: "Despite advertising price cuts, BT are giving 11 million customers a price rise today. Are you one of them? At Sky, we believe in real value and fair and honest pricing - that's why Sky customers can get call packages with Sky Talk that are a lot cheaper than either BT or Virgin Media, with no funny business. To join one of the UK's fastest growing phone services, call us ..."
BT argued that the wording of the ads suggested that it had intentionally misled its customers.
Sky argued that it had been fair and reasonable, but the ASA has now said not so.
The regulator determined that the statements: "We believe in real value and fair and honest pricing" and "no funny business" "went beyond a robust and objective price comparison and implicitly attacked BTs business practices".
"We concluded that the ad was misleading in that respect and denigrated BT", said the ASA.
Sky has been told not to use this approach again.