(Adds reaction, detail) OSLO, Dec 15 (Reuters) - Norway’s top three food retailers face fines totalling a record 21 billion Norwegian crowns ($2.4 billion) for breaching antitrust rules, its Competition Authority said on Tuesday. Norgesgruppen, Coop and Rema 1000 have cooperated in ways that may have resulted in inflated prices, the watchdog said. If confirmed, the combined fine would be the largest ever sought by the regulator in any antitrust case. The three have each employed so-called price hunters to survey stores operated by competitors, and similarly agreed to allow those surveyors access to their own stores, which they said resulted in fierce competition and lower prices. But the regulator said the arrangement had instead probably stifled competition. “Our investigation has shown that the practice of scanning prices at each other’s stores may have resulted in the companies jointly pushing up prices,” the regulator said in a statement. Norgesgruppen’s grocery stores have a market share of about 44% in Norway, while Coop has close to 30% and Rema 23%, the latest data from market research firm Nielsen shows. “Norgesgruppen is categorically rejecting the competition authority’s claim. The industry norm and the so called ‘price hunters’ have lead to lower prices and been favourable for Norwegian consumer interests,” it said in a statement. The company plans to initiate further dialogue with the watchdog, it added. A Rema 1000 spokesman said the company would first read the 280-page report before responding to the claims. Coop did not immediately respond to requests for comment. Norgesgruppen faces a fine of 8.8 billion crowns, while Rema could be fined 7.4 billion crowns and Coop 4.8 billion crowns. The decision to fine the firms is preliminary, in accordance with the watchdog’s practice, allowing each firm to respond to the claims by Apr. 15 of next year. ($1 = 8.7420 Norwegian crowns) (Reporting by Terje Solsvik and Nerijus Adomaitis, editing by Gwladys Fouche and Alexander Smith)