Customer Singapore Airlines says the passenger jet, the world's largest, was worth the wait, and the delivery marks a badly needed morale-boosting milestone for Airbus.

Singapore Airlines Chief Executive Chew Choon Seng said his airline was inconvenienced by the late delivery, but added, "We are glad that Airbus took the time to make sure that the plane is fully tested and developed before it enters commercial service."

For Airbus, big challenges with the problem-ridden plane still lie ahead -- not least producing enough of them.

"Increasing A380 production to meet demand remains our greatest challenge for the next years," Chief Executive Thomas Enders said at a handover ceremony at Airbus' headquarters in Toulouse, southwestern France.

After delivering the first four superjumbos to Singapore Airlines, Airbus will have to redesign cabins and electrical layouts for Emirates Airlines and Qantas. It is committed to handing over 13 planes in 2008, 25 in 2009, and 45 in 2010.

Asked if he was confident that Airbus is up to the challenge, Enders said: "We have every confidence we can deliver, but what is guaranteed in life?"

Lack of a sure-thing might not go down well with Airbus' 16 customers for the A380, whose patience has already been stretched, nor with potential converts. With 189 orders or firm commitments, Airbus is hoping to see 200 on its books by year-end.

Meanwhile, it looks like Airbus is having problems with its next big project -- the A400M military cargo plane.

Tom Williams, Airbus executive vice-president for programs, said difficulties with the engine could push the first flight of the turbo-prop back six months. If the European planemaker is late delivering, "clearly it's not going to be cheap," he told journalists.

US rival Boeing Co is late too, announcing a six-month delay last week to its hot-selling 787 Dreamliner. But the Chicago-based planemaker still has a five-year lead over Airbus for its competing midsize jet, the A350 XWB, which has been set back by multiple redesigns.

Airbus has already been hit with penalties for late delivery of the A380, which combined with spiraling development costs wiped billions of dollars off profits. Enders refused Monday to divulge the extent of the losses.