Customers have been battling against engaged tones to access the £10 a month service, called Surfunlimited. This is despite Telewest telling vnunet.com earlier this week that it was confident it could keep up with demand.

Telewest's problems send out a harsh warning to BT and other telcos planning to launch unmetered internet services this year to prepare for high demand. Previous unmetered Internet services, such as screaming.net, have had similar teething problems.

The Surfunlimited problem is not with Telewest's modems, which have not yet hit capacity, but with the telco's phone exchanges, according to reliable sources. Worse still, calls that cannot pass through the Telewest exchanges are being routed through other operators' networks and in turn are jamming their exchanges.

Telewest is taking urgent measures to increase the capacity of its exchanges. Heads are also believed to be rolling within the company over the bad preparation.

Despite the technical problems, the launch of Surfunlimited has boosted Telewest's internet customer base from about 50,000 to in excess of 135,000, according to unofficial estimates.

"We're absolutely delighted with the response," said a Telewest spokeswoman. "The demand we've seen has been unprecedented. It's testing our service and we apologise to customers for any inconvenience."

"Due to the great response we have increased capacity. Connections should be fine."

Researcher Durlacher said in a report released this week that UK internet use would treble if online time was unmetered. Sarah Skinner, an analyst at Durlacher, said Telewest's problems are similar to those experienced in Silicon Valley.

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"You get what you pay for. I think customers would be happier if [ISPs] said you may not get in first time," she said.

Customers who cannot connect to their local point of presence (PoP) can try dialing into any of Telewest's other PoPs in the UK at no extra charge.

One Surfunlimited customer who contacted vnunet.com confirmed the problem.

"Perhaps Telewest's confidence in its capacity needs to be checked. The past two evenings I have not been able to connect before 10pm and the help line is continuously engaged," he said.

Sources within Telewest confirmed that the customer helpline is frequently engaged. They said one of the main problems is that the lines are being jammed by sales calls.

Earlier this week Telewest said it had spent more than two months updating its infrastructure in preparation for the service. The company's franchise - the area of the country it is allowed to offer services under its operating licence - covers a potential 4.6 million homes, of which the operator says it has signed up about 1.3 million as cable television or phone customers.