The court of appeal has rejected BT's application for retrospective cuts to charges by mobile operators to connect calls to their networks to win back hundreds of millions of pounds.
A tribunal ruled that telecoms regulator Ofcom should have asked for steeper cuts in plans for a reduction over four years announced in 2007, and approved steeper cuts from April 2009.
BT has been calling for the steeper cuts to be applied retrospectively from 2007, but the appeal court found that the tribunal had no power to apply the April 2009 ruling retrospectively.
The telecoms giant pays the mobile operators large sums of money for connecting calls made by its fixed-line customers, but gets smaller sums for connecting calls to its network because its charges are lower, according to the Financial Times.
BT is not expected to appeal and has said in a statement that the decision by the appeal court will not affect the company's financial guidance.
Earlier in April, Ofcom outlined plans for mobile operators to reduce their connection charges over four years to 2015.