The Thai Airways International president has apologised to two passengers forced to give up their first-class seats for two off-duty pilots flying from Zurich to Bangkok.

The aircraft's departure was reportedly delayed for about two hours, with the on-duty pilots refusing to take off until first-class seats were made available for their off-duty colleagues.  

The national airline posted the apology from president Sumeth Damrongchaitham on its Facebook page late on Thursday night.

He said the matter was severe and affected the airline's image. He had ordered an investigation and would ensure such an incident did not occur again. 

"I express sorrow and apologise to all passengers affected by the unprofessional action that caused the delay. And I apologise to the passengers who were directly affected by the seat change. I take responsibility for the incident," Mr Sumeth said.

The incident reportedly involved THAI flight TG971 from Zurich to Bangkok on Oct 11.  It was scheduled to depart at 1.30pm but the duty pilots refused to take off unless some first-class passengers agreed to give up their seats to two THAI pilots deadheading - flying to a destination to start work - to Bangkok.

All first-class passengers initially refused to do so. After about two hours of delay, a couple finally vacated their seats so the flight could depart. 

They later filed a complaint, saying the two off-duty pilots should have been given vacant seats in business class, rather than taking all passengers hostage to achieve what they wanted.

The flight was over an hour late arriving at Suvarnabhumi airport.