The Swiss Federal Administrative Court concluded Swisscom had behaved improperly in the 2008 tendering process for the broadband networking of Swiss Post sites, largely upholding COMCO's decision though reducing its fine to 7.4 million Swiss francs ($8.05 million), from the original 7.9 million franc COMCO fine.
Sunrise claims it was impossible for it to submit a competitive tender because it was dependent on wholesale telecoms products provided by Swisscom for which the state-controlled telecoms company demanded too much money.
"The high prices charged by Swisscom for those preliminaries made it impossible for Sunrise to submit a competitive tender," the Federal Administrative Court said in a statement.
Swisscom contends that Sunrise, since bought by Liberty Global, would have been in a position to submit a competitive bid and plans to take its fight to the country's highest court.
"Swisscom will lodge an appeal against the judgement with the Federal Supreme Court," it said.
"Swisscom remains of the opinion that its behaviour in this tendering process was entirely in accordance with the law," Swisscom said in a statement.
($1 = 0.9194 Swiss francs)
(Reporting by John Miller;Editing by Elaine Hardcastle)