The ad showed Willis complaining about the speed of his current provider and a character saying: "You could try Sky Broadband, it's totally unlimited", and "Sky Broadband Unlimited is £7.50 a month, which is less than half BT's standard price."
On-screen text stated: "Price for Sky TV customers. Sky Talk and line rental (£14.50 pm) required."
A viewer complained that the ad was misleading because it did not make clear the extent of the commitment consumers had to make in order to obtain the £7.50 deal.
Defending the ad, Sky said the customers were only required to take Sky line rental for £14.50 a month.
But Sky added that the deal cost £7.50 for Sky TV customers and £10 for non-Sky TV customers, with TV deals starting at £21.50 a month.
It said all the necessary information was made clear in on-screen text.
Upholding the complaint, the Advertising Standards Authority (ASA) said small print at the bottom of the screen would be significantly less prominent than a claim made in a voice-over, and was therefore not an appropriate method of communicating material information relating to the £7.50 price claim.
The ASA said: "In order to ensure they were sufficiently prominent, we considered that the minimum monthly price of both Sky TV and line rental should have been clearly communicated together with the £7.50 price claim.
"Because it was not, we concluded that the ad did not make sufficiently clear the extent of the commitment consumers had to make in order to obtain the broadband service at the advertised price of £7.50 per month, and was misleading."
It ruled that the ad must not appear again in its current form.