The television advert, which said Internet on the phone was "really fast", showed a close- download a file, with waiting times of only a fraction of a second.

But 17 viewers complained to the ASA, saying the advert was misleading as they believed it exaggerated the speed of the iPhone 3G.

Third-generation, or 3G technology enables users to access broadband Internet on their mobile phone.

In response to the complaints, Apple (UK) Ltd said the claims made in the advert were "relative rather than absolute", and compared the new 3G iPhone with its 2G predecessor.

The ASA report said Apple pointed out that as mobile devices using 3G technology operated at substantially faster speeds, the claim was not misleading.

It also said Apple maintained "the average viewer would understand that a 30-second TV ad ... was merely simplified to allow an illustration of the device".

But the ASA ruled the advert must not appear again in the same form, as many viewers might not be fully aware of the technical differences between different types of technology.

Although the advert included an on-screen disclaimer which stated "network performance will vary by location", the ASA said the pictures, combined with the repeated use of the claim "really fast" were likely to lead viewers to believe the phone worked at or near the speeds shown.

"Because we understood that it did not, we concluded that the ad was likely to mislead," the ASA added.

The iPhone, which combines a phone with a music and video player, was first launched in mid-2007. The 3G model was voted gadget of the year by readers of Stuff magazine in September.