The Thai Airways International (THAI) workers' union has attacked the airline's plan to buy or lease 38 new aircraft, saying it would put further strain on the company that is already suffering under crippling debt.

Union president, Damrong Waikanee, on Monday said the planned purchase or leasing of new planes, estimated to cost 130 billion baht, would add insult to financial injury for the national carrier which is facing accumulated debt exceeding 100 billion baht.

Staff were eager to know how the investment would improve the company's balance sheet, for example on what routes the new planes would be flown and what criteria would be adopted to evaluate the investment worthiness of the proposed aircraft purchase.

THAI president Sumeth Damrongchaitham said in May the aircraft acquisition plan was being put to the cabinet for approval.

There would be two phases. The first stage is for 31 planes to replace old aircraft within five years, and the second would be for seven other aircraft, Mr Sumeth said.

Mr Damrong said THAI should have learned a lesson after it bought a certain type of aircraft which used a lot of fuel and was not fully utilised. The planes were grounded and had to be sold off at a heavy discount.