Wanadoo has been criticised for adverts which failed to show that one of its fast broadband schemes was only available to a limited number of users.
The Advertising Standards Authority (ASA) found that when the adverts first appeared, the firm's 8 megabit service could only be used by 4.7% of people.
The high speed depended on upgrades to users' local phone exchanges which had only been carried out in a few areas.
Wanadoo said it had now "slightly tweaked" its adverts.
'Fastest possible'
In its ruling, the ASA also found that the firm had failed to show in its advertisements whether the 8 megabit speed referred to downloading or uploading, or both.
Wanadoo was also criticised for failing to show that certain services were only free for the first six months of a user's contract.
"We have slightly tweaked our advertising as a result of the ASA ruling," said Wanadoo.
"Our promise is, and always has been, to give customers the fastest speed that their line can currently handle with a free upgrade to the faster speeds once their local exchange has been upgraded by Wanadoo."
Wanadoo is one of the UK's most popular broadband providers.
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