The ads, which ran on TV, print, outdoor and online, attracted 26 complaints in total. Twenty-four disputed BT Sport was "free" and two challenged whether claims about the cost of the package and the contract were misleading.
The TV spot, created by Abbott Mead Vickers BBDO, portrayed a TV producer trying to convince famous footballers, such as Gareth Bale and David Luiz, to wear a zero on the back of their team jerseys.
The on-screen text said: "Free for existing BT TV customers when you re-contract TV & BB for 18 months."
A voiceover later said: "The Uefa Champions League, free for our BT TV customers", as on-screen text in the final shot said the same.
Other campaign executions included a poster headlined "Watch the Uefa Champions League for free", an email ad that described the offer to customers, and a press ad with the headline, "BT Sport Europe Free with BT TV".
When contacted by the Advertising Standards Authortity, BT said BT Sport was provided to TV customers who opted to take it at no extra cost with a one-month minimum term. It could be cancelled at any time with 30 days’ notice and details of the notice period were provided when consumers bought BT broadband.
They added that BT Sport was free with BT TV, on a conditional purchase basis. If consumers took BT TV, they could opt-in and add the BT Sport pack to their package at no additional cost.
For the TV ad, Clearcast said BT Sport was a conditional add-on that did not have to be taken with BT TV. They believed the on-screen text made that clear and that the "free" claims were therefore not misleading.
But, in its ruling published today, the ASA said the ads must not be shown again.
The watchdog said: "We considered it was likely to be understood by consumers to mean that if they wished to view Champions League games, they would need to renew their BT TV and broadband contract for 18 months or alternatively pay £5 per month.
"We noted it was not possible to continue to view BT Sport without paying the additional monthly fee or re-contracting and acknowledged the main body of the ad did not state that consumers had the option to cancel their BT Sport pack without the need pay extra or have to extend their contract."