The case was brought against Royal Mail and regulator Postcomm by consumer group Postwatch. The ruling could result in 40m of extra compensation being paid to corporate customers for 2003-2004, with around 15m for 2004-2005.

"We expect to ask leave to appeal the ruling," said a Royal Mail spokesman. "It is too early to say what will be the impact of the ruling for our business and bulk mail customers."

The case stemmed from a compensation clause in the Royal Mail's postal licence, interpreted for it by Postcomm, which Postwatch argued had allowed the carrier to under-compensate users of bulk mail.

"The compensation scheme, written by Postcomm, is the best of its kind in the postal world," added the Royal Mail spokesman. "It was applied by Royal Mail fully and fairly."

"We need to read a full transcript of the judge's ruling and we are not likely to see that until after Christmas," said a Postcomm spokesman.