Hong Kong flag carrier Cathay Pacific is to buy budget airline HK Express for over US$600m as it moves to counter competition from the increasing number of low-cost carriers in the region. 

The move is its first foray into the budget sector.

It will leave Cathay controlling three of the four airlines at one of Asia's busiest airports at a time of huge growth in the region's air industry. 

It comes weeks after the carrier said it had swung back into the black in 2018 after two years of losses.

The deal will also help ease concerns after an embarrassing data breach that dented the firm's reputation and could prove costly.  

"HK Express captures a unique market segment," Cathay said in a statement. 

"This represents an attractive and practical way for the Cathay Group to support the long-term development and growth of our aviation business and to enhance the competitiveness of the Hong Kong hub during a time of intense regional competition." 

Cathay will stump up HK$4.93 billion (US$628.15 million) for the airline in a deal that is expected to complete by December, according to a filing announcing the deal.

The firm said it will continue to operate HK Express as a "standalone airline using the low-cost carrier business model".