Suppliers will have to provide information at the point of sale detailing deficiencies, such as the inability to call 999, the phone line going dead in power cuts and the service not working when the buyer’s computer is turned off.

Despite opposition from the companies providing the high-tech service, Ofcom, the telecoms regulator, is understood to be preparing to forge ahead with plans for a mandatory code of practice. An announcement is expected within the next couple of weeks.

Customers will have to sign to say that they understand the conditions when they buy Voice over Internet Protocol (VoIP) services. Labels on product equipment will carry the warnings.

It is thought that services such as Skype — where customers do not buy a product in a store but download the necessary software online — will have to display such warnings onscreen.

The providers may also be required to tell consumers about the unavailability of facilities that they might expect from a telephone service, such as an answering machine.

Nearly two million British households have services that allow them to make calls over the internet and Ofcom believes that the conditions are ripe for a significant increase in uptake over coming years.

The services enable customers to call relatives and friends worldwide for free — or at least for much less than they would expect to pay with a traditional landline.

So far, the regulator has sought to stand back from internet calls and give the technology space to develop.

However, as the services have become more of a mass-market proposition — the market is now worth up to an estimated £80 million — it is keen to ensure that consumers fully understand the products and are protected from any abuse.

Many of the call providers, including Skype, which has about 120 million registered users, condemned Ofcom’s proposals when they were put out for consultation as a form of “super consumer protection” that other telecoms groups do not have contend with.

Yesterday the Internet Telephony Services Providers’ Association said that it had “major concerns”. A spokesman for the group, whose members include BT and Vonage, said that the rules could force providers to go to other countries where they would not be hit.

Eli Katz, its chairman, said: “These proposals are the most prescriptive set of regulations of anywere in the world.’’

A spokesman for Ofcom said the watchdog intended to announce its final plans on the consultations before Christmas.

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