The roll out of Thai Airways International's brand-new Airbus A350 XWB jets has been put on hold.
The flag carrier had planned to debut the state-of-the-art aircraft on its Bangkok-Melbourne route as part of THAI's enhanced service.
But Australian civil aviation authorities have kept THAI in the dark as to whether or when it will approve the airline's application to operate the world's latest commercial airliner to and from Australia.
The authorities were cited as saying they were "unfamiliar" with this type of aircraft, especially on issues related to airworthiness, insiders with knowledge of the matter told the Bangkok Post.
The aircraft received certification from the European Aviation Safety Agency in September 2014, with certification from the US Federal Aviation Administration coming two months later.
No airlines have operated this type of European-made aircraft over Australian airspace, and THAI would be the first international carrier to do so if permission were granted.
THAI had planned to start flights of its first A350 XWB on the Melbourne route on Sept 16, two weeks after the first of 12 such aircraft was delivered from Airbus's final assembly plant in Toulouse.
The first A350 XWB entered commercial service with Qatar Airways on Jan 15, 2015.
To date, there are over 42 A350 aircraft which have been delivered to eight operators, including Singapore Airlines, Cathay Pacific and Vietnam Airlines.
As of September 2016, Airbus had received 810 orders for A350 XWBs from 43 customers worldwide.
"We are at a loss and absolutely have no idea why it is taking so long for them [Australian officials] to consider our application," said a THAI executive who asked not to be named.
He added that the 60-day period to consider the application had already passed.
Uncertainty over the matter and the arrival of THAI's second A350 at Suvarnabhumi airport this week have prompted the flag carrier to reactivate its Plan B for the aircraft.
THAI will now deploy its first pair of long-range twin-engine wide-body jet airliners on its European trunk routes, instead of flying to Australia.
The airline has already sought permission to operate the new aircraft from the European civil aviation authorities, whose endorsement is expected shortly.
THAI expects to start operating the A350s on non-stop flights from Bangkok to major European cities such as London, Paris and Frankfurt by Oct 28.
The A350's long-range capability will make the switchover easy, while its technological advancement and passenger appeal will support THAI's marketing efforts in Europe.
Before the European launch, the airline will have the first two A350s operate "run-in" flights to popular domestic and regional routes such as those from Bangkok to Chiang Mai, Phuket and Singapore, according to insiders.
With Australian approval still pending, THAI will continue to operate Boeing 777-300ERs on its twice-daily Melbourne services.