The decision is a blow to BT, as the need to change numbers has stopped many people from switching to a rival telephone firm.
Businesses in particular are deterred because of the costs of reprinting stationery and changing advertisements. BT said the change would technically be 'a massive operation' and might not be best for customers.
Initially, number portability between operators will be possible only if customers stay at the same address. However, there are plans whereby customers may take their numbers with them when they move. Eventually they will be able to retain one number from cradle to grave.
Don Cruickshank, director-general of Oftel, said that removing the inconvenience of changing numbers meant that customers could in future base their choice of operator purely on price and quality of service.
How quickly it took effect depended on the operators.
Many cable television companies offer telephone services in their franchise areas and have been lobbying for the change. The companies undercut BT prices by between 10 and 20 per cent and are expected to more than double their number of subscribers to about 700,000 by the end of this year.
A spokesman for BT said: 'We do not object to anything that benefits customers, but we might possibly question that this benefits customers.
'It cannot happen overnight, and if someone comes to us tomorrow or next month we would say that if you leave BT you leave your number behind.'
He added that BT must also work out the cost, which the company hoped would not be passed through to customers.Reuse content