State-owned Airports of Thailand Pcl (AOT), operator of the key Suvarnabhumi airport and low-cost Don Muang airport, should hold multiple concessions by categories to maximize benefits, the Thai Duty Free Shop Trade Association and the Thai Retailers Association said.

"A master concession ... is monopolistic system which does not bring fair competition," the groups said.

AOT has previously said it was considering four types of concessions, including master and multiple concessions.

The auctions were supposed to be held last month, but they have been delayed. It is not clear when they will be held next.

The lucrative duty-free concession at Suvarnabhumi is currently held by unlisted King Power, whose chairman died last year in a helicopter crash.

Thailand's tourism boom, which saw 34.4 million arrivals between January and November last year, mostly go through airports with King Power stores. But public debate has increased over the single duty-free operator structure.

King Power's single licence is set to expire in 2020, with rivals including Central Group, the Mall Group and South Korea's Hotel Shilla interested in bidding, analysts say.

The retail groups also proposed that public pick-up counters at the airport for goods purchased in downtown duty-free shops should be managed by AOT. The counters are currently operated by King Power under its concession.

(Reporting by Chayut Setboonsarng, additional reporting by Panu Wongcha-um; Editing by Himani Sarkar)