Until now, as BT owns the customer's telephone line, it has been able to launch last-ditch marketing efforts to try to persuade customers not to switch, as the alternative provider is obliged to notify BT when a customer signs up to it.
Oftel has been forced to act following complaints from BT's rivals, including Scottish telecoms group Thus. It has the power to impose significant fines should BT continue the practice under new communications legislation.
BT has rounded on Oftel over the decision, which it has described as "ill-conceived". Pierre Danon, chief executive of BT Retail, warned that it could lead to customers being switched to an alternative telecoms provider without their knowledge or consent.
"Phone customers are only just getting over the confusion caused by Oftel's handling of the 192 switch-off. Now they'll be getting more of the same," he added.
BT is planning to appeal against the decision.