Royal Mail is to continue to appeal over a £50 million fine from Ofcom after a tribunal upheld the punishment last month.
The delivery firm said it had asked for permission to challenge the Competition Appeal Tribunal decision in the Court of Appeal.
The fine was initially handed down by the communications regulator in August last year, after it found that Royal Mail had discriminated against one of its biggest rivals, Whistl.
In 2014, when Whistl was known as TNT, the company tried to become the first to seriously rival Royal Mail in delivering letters.
However, the formerly state-owned business, which had been privatised the year before, used its dominant position to discriminate against the fast-growing rival.
Royal Mail later appealed against the £50 million penalty and, after the tribunal ruling last month, said it was still considering its legal options.
On Wednesday, the company said: “Royal Mail plc has lodged papers with the Competition Appeal Tribunal to seek permission to appeal its recent judgment on Royal Mail plc vs Office of Communications (Ofcom) to the Court of Appeal.”
After last month’s judgment, an Ofcom spokesman 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.
“We hope that our fine, which has been upheld in full by the tribunal, will ensure that Royal Mail and other powerful companies take their legal duties very seriously.”