21CN will now be implemented over three years from 2008, according to information on the company's Web site. This shows that some parts of Northern England, Scotland, Wales and South West England will not have been migrated to the all-IP network by 2010.

Nationwide rollout will only begin after a review of early installations, which is intended to take place early in 2008. Back when the project was first announced in June 2004, BT had said that "mass migration of customers onto the new network will begin in 2006".

"We always said it would take time — it is an ambitious programme," said a BT spokesperson on Tuesday. "21CN covers 30 million lines and it really can't be done any quicker than this." Despite the overrun, BT is insistent that the network will be completed within the original budget (£10bn) and with the projected cost savings.