* Qatar Airways may shift demand to A350-1000 from sister model
* Largest member of A350 family delivered to biggest customer
* Airbus favours existing models, no plans to add bigger version (Recasts after briefings)
By Alexander Cornwell
TOULOUSE, France, Feb 20 (Reuters) - Airbus got a boost for its largest twin-engined jet on Tuesday when Qatar Airways said it was considering upgrading some of its A350 orders to the largest model as it took delivery of the first such plane.
The A350-1000 is designed to seat 366 people and competes head-to-head with Boeing's profitable 777. The first A350-1000 was handed over to the Gulf carrier on Tuesday, joining the smaller A350-900, which has been in service for three years.
Airbus says the lightweight A350-1000 is 25 percent more efficient than the most popular current version of the 777, the 777-300ER.
But sales of the 777-300ER have picked up, and Boeing is working on plans to leapfrog the A350-1000 with an upgraded 777X boasting over 400 seats.
Boeing last year sold 32 777-300ERs against just one order for the A350-1000. Some airlines have begun downgrading some A350-1000 orders to the 325-seat A350-900.
Qatar Airways, which has ordered both the A350-1000 and the 777X, indicated it was moving in the opposite direction and said it could shift more of its A350 orders to the largest model.
"There is a possibility that we could convert some of the 900s to the 1000," Chief Executive Akbar Al Baker said.
Qatar is the top A350 customer with 76 on order, including 37 A350-1000s, which have a list price of $367 million.
It recently cancelled four A350-900 orders following delays, but subsequently re-committed to the new European jet family.
The prospect of upgrades will come as a relief to Airbus, which is gambling on the A350-1000 to contain any market pressure from the 777 as Boeing develops its new model.
Last summer, the European planemaker shelved tentative plans for an even bigger A350 that would compete more directly with Boeing's planned 777X.
Fabrice Bregier, speaking on his last day as Airbus chief operating officer, said on Tuesday studies had shown the idea worked in principle, but that Airbus would focus instead on pushing the A350-1000.
"It's now time to start to be more aggressive and to explain to our customers, or Boeing's customers, that this aircraft will be a better choice than a 777-9X," Bregier said.
Boeing insists that its jet will be the world's most efficient aircraft in its category, thanks to new wings.
Baker said Qatar Airways, one of the world's major fleet buyers, is not interested in an ultra-long-haul version of the A350-900 being floated by Airbus for carriers like Qantas but could buy more of regular A350 jets.
"Yes, there may in future be a requirement for more of these airplanes for Qatar Airways, especially when we do further enhancements of our acquisitions," he said.
"And of course there is a probability we will buy more of these airplanes to put in our leasing company."
He also ruled out orders for the largest and smallest Airbus jets - whether the 544-seat A380, of which it has bought 10 and has options for another three, or the Bombardier CSeries, a 110 to 130-seater that Airbus agreed to rescue last year.
Writing by Alexander Cornwell, Tim Hepher; Editing by Larry King