The existing duty-free licence is set to expire in 2020, and AOT postponed the auction process earlier this month due to public concern over the perceived monopolistic structure of the bidding process and the concession.

The duty-free business is a major beneficiary of a tourism boom in Thailand, where arrivals exceeded 38 million people in 2018.

King Power Group, owner of the English Premier League's Leicester City Football Club, currently holds the licences for duty-free retail and commercial activity at Suvarnabhumi, the country's main international airport, and also at airports in Chiang Mai, Hat Yai and Phuket.

Under a new arrangement, AOT has divided the new concession into two separate contracts - one for Suvarnabhumi which will be open for bidding in May and another contract covering Chiang Mai, Hat Yai, and Phuket airports that will not be open for bidding yet.

Another licence up for auction in May is for non-duty-free commercial activity at Suvarnabhumi, which also expires in 2020.

AOT earned 16.7 billion baht ($526.2 million) from concessions in its last fiscal year, 13.3 percent more than a year earlier.

The concession will start on September 28, 2020 and will expire on March 31, 2031, AOT said in a statement.

The bidding will take place on May 22 and results will be announced by May 31.

(Reporting by Panarat Thepgumpanat and Panu Wongcha-um; Editing by Gopakumar Warrier)