The purchase allows King Power's billionaire owner Vichai Srivaddhanaprabha to further tap into Thailand's booming tourism industry and expand beyond his duty-free and hotel portfolio.
"We are in the same business, the tourism business, which is a key factor in growing the Thai economy," Vichai told reporters, adding that he expects Thai Air Asia to continue growing by 20-25 percent a year.
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Thai Air Asia is 55 percent owned by Asia Aviation, a Thai company, while the rest is owned by the Air Asia group founded by Malaysian entrepreneur Tony Fernandes.
King Power has purchased 39 percent of Asia Aviation from its CEO Tassapon Bijleveld for 7.945 billion baht, making it the second largest shareholder in Thai Air Asia.
Bijleveld will keep five percent of the shares and stay on as CEO, King Power said in its statement, with the rest of Asia Aviation's stock owned publicly.
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In its statement announcing the deal, King Power said it was able to snap up the shares at a price "substantially lower" than the market rate because local airlines must legally be more than 50 percent owned by Thais.
"Given the restrictive nature of such rules, there are only a handful of Thai individuals who, realistically, would be interested in and could afford the purchase," the statement said.
Asia Aviation's share price was up 1.67 percent to 6.10 baht by the lunchtime break.
Polo-loving Vichai, 58, has carefully navigated Thailand's treacherous political waters of recent years, while taking his King Power empire from strength to strength.
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He has built an estimated fortune of $2.9 billion since establishing the firm in 1989 -- starting with a single shop in Bangkok.
He hit the jackpot in 2006 when King Power won a monopoly duty-free concession at Bangkok's cavernous new Suvarnabhumi airport, and with it a captive market of millions of travellers.
He has since gone on to buy Leicester City and steer it towards this season's remarkable Premier League victory.
Last year Thai Air Asia carried 14.8 million passengers and plans to serve 16.9 million travellers in 2016. It made $30 million net profit last year, a company record.
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