Royal Mail has failed to have a record £50m fine from Ofcom overturned.
The fine, announced in August 2018, related to its actions in 2014 when Whistl, then known as TNT, was trying to become its first competitor in wholesale mail delivery.
Ofcom’s investigation followed a complaint by Whistl that Royal Mail had abused its dominant market position.
Royal Mail challenged the fine, but on Tuesday, the Competition Appeal Tribunal dismissed its application.
An Ofcom spokesperson said: “We found that Royal Mail pursued a deliberate strategy of pricing discrimination against Whistl, which was its only major competitor for delivering business mail.
“Royal Mail had a special responsibility to ensure its behaviour was not anti-competitive.