Mercury wants BT's charges for making the link to be completely restructured. It says that it needs more flexibility to be able to offer customers special packages which could include free off-peak calls.
The action comes only weeks after an Oftel ruling on interconnection which will result in BT paying Mercury up to pounds 90m. However, Mercury said that in making the ruling, Oftel missed a chance of overhauling the entire structure of interconnection payments between BT other operators.
Mercury said that it should be able to give BT an upfront payment to cover the costs of providing interconnection. But Oftel considers that the payment should be made on a pence per minute basis.
Mike Harris, Mercury's chief executive, said: 'We are determined to use every means possible to break BT's dominance. On this occasion it means using the courts.'
He said that the challenge was not a judicial review of the recent interconnection determination by Oftel, but that it aimed to seek clarification of BT's licence as regarded the future structure of prices for interconnection.
Oftel said it had already asked Mercury to put forward a paper on the charges for the link-up but had received nothing. Don Cruickshank, Oftel's director general, said that call charges set by Mercury and others should not be restrained by BT's own tariff policies and that he was reviewing this issue.
Mercury said that Oftel's stance prevented it from offering deals such as free offpeak calls, which its mobile subsidiary, One-2-One is already doing in the London area.
A spokesman for the company said: 'Timing is all-important. If we can get clarification, then we move forward immediately on pricing.'
He added thatOne-2-One had been an 'illuminating success'.