Under current arrangements for some types of services, notably carrier pre-selection, a telecoms company has to notify BT when a customer intends to switch to its service, so that BT can make the necessary changes to its own systems to allow the transfer to take place.
Until now, BT has passed this information to its marketing department, which has then contacted the customer to try and persuade them to stay with BT.
Following a complaint received from Thus Plc and Broadsystem Ventures Limited, Oftel launched an investigation. On Friday the telecoms watchdog ordered BT to stop the practice, on the grounds that it is breach of the new General Conditions made under the Communications Act 2003 that came into force in the UK in July.
The requirement affects BT when its customers switch to alternative carrier pre-selection operators, but will also apply to alternative service providers to prevent them from contacting their customers who want to switch to another operator.
BT has condemned the decision, stating that it will leave customers confused, and may leave them vulnerable to slamming - where a customer is switched to an alternative supplier without their knowledge.
But Oftel said that both the gaining and losing carrier pre-selection service providers still have to write to customers notifying them of the change. This will give customers the opportunity to cancel the change before it has taken place if it has been attempted without their prior consent.
David Edmonds, Director General of Telecommunications said on Friday:
"BT are entitled to target lapsed customers to encourage them to return to BT, but it cannot act upon the transfer notification received from alternative operators to carry out this activity."