Ofcom said that by the end of the year, there could be as many as 3 million VoIP users in the UK as companies such as Skype, BT and Vonage add more users. Most subscribers use the service as a secondary telecoms line within the home.

The new rules follow a public consultation last year that was intended to help consumers make more informed decisions about VOIP services.

"Providers that don't offer emergency service calls and don't work without a power supply must display warnings on their products." – Ofcom.

"VOIP providers will also have to disclose whether customers can keep their same phone number if they switch providers." – Ofcom.

“Operators must be clear about whether their service will work in the event of a power failure." - Ofcom.

Many VOIP services depend on a powered router for Internet access, so if the home power supply were cut off, the VOIP service also would not work.

So far, the European Commission has taken a ‘light touch’ attitude towards VOIP and not imposed regulation as the industry matures a stance it adopted in 2005.

In the U.S., the Federal Communications Commission has banned companies from advertising VOIP services if they do not allow people to call emergency services.

"In assessing the need for any new requirement, we will continue to carefully consider the impact of such regulation on market entry, innovation and competition," Ofcom said on its Web site.

David Quirk, CEO of C21 Communications said: “The concern for public safety is more than welcome, but regulations should take into consideration that VoIP services already cross boundaries. Nowadays UK customers have VoIP services from many different operators all over the world and competition might become unfair for UK operators facing strict regulations."

VoIP-only providers argued that Ofcom could place an unnecessary burden on the emerging industry and should instead focus on emerging technological trends such as "naked DSL", where consumers can source broadband without paying line rental.

ITSPA - the VoIP provider trade body - said it feared the code could trigger a stricter regulatory framework for VoIP than for other technologies used within the telecoms industry and that overburdening small service providers with regulatory requirements could hinder creative development.