BT has been ordered to stop using "dirty tricks" to persuade customers from switching phone providers.

In a ruling published today, Oftel upheld a complaint from Thus and Broadsystem Ventures Ltd, and so preventing BT from using information about the transfer of customers to alternative telecoms suppliers such as One.Tel, Tiscali and Tele2.

Oftel found that the UK's dominant telco was using this information for illegal marketing activity.

In a statement Oftel explained: "Until now, BT has passed this [transfer] information to its marketing department, which has then contacted the customer to try and persuade them to stay with BT. Oftel has today ordered BT to stop carrying out this practice, on the grounds that it is forbidden under the new EU Access and Interconnection Directive that came into force in the UK in July 2003."

Alternative telco Thus said it welcomed Oftel's move against "BT dirty tricks" and urged the regulator - soon to be replaced by new communications regulator Ofcom - to "remain vigilant" about BT's competitive behaviour.

But the monster telco lashed out at the ruling and vowed to appeal. It condemned Oftel's move as an "ill-conceived move that will cause widespread confusion for customers".

It warned that unless BT is able to contact customers, it could open the floodgates for 'slamming', a process where customers are switched phone providers without their knowledge or consent.

Said BT Retail chief exec Pierre Danon in a statement: "This decision makes no sense for the customer. Phone customers are only just getting over the confusion caused by Oftel's handling of the 192 switch-off. Now they will be getting more of the same.

"This will break an important link with our customers, which will not be in their best interest or ours. We will not be able to call a customer until they have left and they may not be clear which services have been terminated. The potential for confusion is huge." ®