Virgin Media has been reprimanded by regulators for misleading consumers about its broadband speeds in a recent advertising campaign.

In response to a complaint by Virgin's bitter rival BSkyB, the Advertising Standards Authority will rule on Wednesday that Virgin's print and poster advertisements did not make sufficiently clear that customers could experience connections below the advertised 20 megabits per second.

The ASA went further than most adjudications, which typically ban the offending advertisements. In addition, it has told Virgin to ensure future advertisements for that package included a disclaimer - in the main copy, not just the small print - that its speeds could be affected by the number of users on the network.

Ofcom, the media regulator, is conducting a large-scale survey of UK broadband performance due to ongoing concerns that actual speeds lag the services advertised.

In June, it published a voluntary code of practice which requires broadband companies to provide customers with estimates of the speed their individual phone lines can support at the point of sale. Virgin, Sky and the other main broadband providers such as TalkTalk and BT are signatories to the code.

A Sky spokesperson said: "We welcome the ASA ruling. Customers deserve transparent information not misleading claims."

Virgin's marketing explicitly compared its cable network to its competitors, who all use copper-wire broaband connections. The ASA ruled that one of Virgin's claims inaccurately implied copper broadband was always slower than cable, and that another suggested cable broadband would never slow down.

Virgin acknowledged speeds on its fibre-optic network slowed as more people were online, as with all broadband lines. But it said cable was not adversely affected by "distance from the exchange and line noise", as its copper-based rivals are.

In spite of the ruling, Virgin spokesperson said that independent sources backed its claim that it was the UK's fastest broadband provider. "We will continue to communicate the advantages of our network, and the speed and reliability of the broadband we deliver, in a way that is clear and transparent to consumers."