Two local operators, NextCall and Localtel, complained that the BT service was inadequate - and now the regulator has agreed.

Oftel says that some customers who tried to switch from BT were left without any telephone service for up to 33 days.

And BT failed to provide sufficient information to the rival telephone companies to enable them to bill around one in ten of their customers.

And up to 30% of customers who then switched back to BT were billed twice for the service.

Oftel says BT is in breach of its license, which prohibits it from discriminating against service providers or unduly preferring its own business.

It is giving BT two months to improve its performance.

Long delay

According to David Edmonds, head of Oftel, despite months of investigation BT has failed to correct the problem.

"The evidence is now clear that BT has failed to alleviate the situation both for the service providers and the customers .. service providers will play an increasingly important part in the development and delivery of telecommunication services. For service providers to satisfy their customers' expectations, they need to be confident that BT will meet their obligations," he said.

BT set up its Call & Access service in October 1998, in which BT maintains the line and quality of the call service, while the rival provider carries out billing and customer service.

Following customer complaints, Oftel has been investigating the problem since May.

Other clashes with Oftel

It is not the first time that BT has clashed with the regulator on access to rival services.

Last month Oftel ordered BT to open its high-speed Internet service, which is plans to roll out in the next six months, to rivals by 2001.

And earlier in the year it was ordered to cut the cost of calling to mobile phones.