Virgin Media has been rapped by the Advertising Standards Authority (ASA) over a recent promotional campaign.
The watchdog ruled that claims made in a national campaign about the broadband speeds homeowners could receive were unsubstantiated.
Rivals BSkyB lodged a complaint suggesting that it would be impossible for Virgin's network to provide all homes with broadband speeds of 20Mb.
The broadcaster said that Virgin lacks the network capacity to supply all subscribers with the maximum speed at the same time.
After seeking a response from Virgin, the ASA upheld the complaint that the claim "10 out of 10 homes with our fibre optic broadband can get 20Mb" is misleading.
"Because we had not seen data demonstrating that all Virgin's 20Mb cable broadband customers could achieve the headline speed in the home, we concluded that Virgin had not substantiated that claim," the advertising regulator said.
The ASA also challenged another line from the same advert which read: "70% of homes in the UK can't get 8Mb over BT phone lines", suggesting the claim was potentially misleading to customers.