Newcastle United's proposal to sell next season's shirts exclusively through JD Sports is "unlawful", rival firm Sports Direct has claimed.
Sports Direct, run by former Newcastle owner Mike Ashley, is seeking an injunction from the Competition Appeal Tribunal to stop the deal.
Company lawyers said preventing the "home of football supplies" selling cheaper shirts would harm fans.
Newcastle United said there was "no evidence" competition would be damaged.
At the tribunal in London, Sports Direct's barrister Tony Singla KC said football was "essential" to the firm, which was the "largest sports retailer in the UK" with 488 stores.