The A350 is currently due to enter service in 2010

The carrier is currently the biggest customer for the forthcoming mid-sized, long-distance plane - a rival to Boeing's new 787 Dreamliner.

But Qatar's chief executive Akbar Al-Baker told the Reuters news agency that he was worried design changes to the A350 would delay its launch date.

Airbus said it would not comment on issues between it and its customers.

'Review the options'

The Airbus A350 and Boeing's 787 Dreamliner are both currently still in development, with the Dreamliner due to enter service in 2008, and the A350 timetabled for 2010.

The Dreamliner is currently proving the more popular, with 291 firm orders compared to 100 for the A350.

Boeing's Dreamliner is set to fly in 2008

After Airbus said last week that it would now be announcing design changes to the A350 sometime in the next two months, some analysts fear its launch may as a result be put back to 2012.

This made it difficult to commit to the purchase, Mr Al-Baker said.

"We have not signed a purchase agreement because we cannot...purchase an airplane which is undefined," he said.

"The definition will change, the performance will change, the fuel burn will change. So many things will change that the aeroplane that we signed for in last September is not there anymore. So we have to, of course, review all our options."

State-owned Qatar Airlines flew 6.3 million passengers in the year to the end of March.

Most of its current fleet of 46 planes are Airbus.