An explosion that blew a hole the size of a small car in a jumbo jet from London to Melbourne was not a terrorist attack, investigators believe.

Frightened passengers on the Qantas Boeing 747 had to wear oxygen masks when the gaping hole appeared in the jet 30,000ft up.

The aircraft made an emergency landing in Manila in the Philippines after the explosion happened on the Hong Kong to Melbourne leg of the journey yesterday.

Air accident investigators are probing the cause of the explosion, but they do not believe it was caused by a terrorist attack.

Initial investigations suggested that a section of fuselage had separated and that there had been an ``explosive decompression''.

As air accident experts investigated, passengers, who spent the night in Manila, flew on to Melbourne in a replacement Qantas Boeing 747.

Ruben Ciron, chief of the Civil Aviation Authority of the Philippines said today that four personnel from the Australian Transport Safety Bureau were still gathering facts about the incident.

Boeing spokeswoman Liz Verdier said it was too soon to determine what caused the hole, but the company was providing technical assistance to the investigation.

``We are dispatching four personnel from Boeing, an investigator and three engineers who are leaving immediately,'' she said.

No-one was injured and all 346 passengers and 19 crew were able to disembark normally after the aeroplane landed.

Video footage taken by passengers on their mobile phones and posted on the internet site Youtube showed a tense quiet punctuated only by a baby's cries as passengers sat with oxygen masks on their faces.

The jerky footage showed a woman holding tightly to the seat in front of her as rapidly approaching land appeared through a window. Loud applause and relieved laughter went up as the plane touched down.

The Boeing 747-400 was manufactured in 1991 and delivered to Qantas in the same year.

Aviation expert Chris Yates said: ``From the pictures coming in from Manila it's quite evident that a section of the fuselage gave way in flight.

``As a consequence of this the aircraft experienced rapid decompression. Fast action from the pilot and co-pilot ensured that all those aboard remained safe.''

He went on: ``Australian air accident investigators will examine closely the fracture points evident on the skin of the aircraft to determine whether metal fatigue or manufacturing defect caused the panel to be ripped away from the remainder of the fuselage in flight.

``This is not an uncommon occurrence, every year there are reports of panels being lost from aircraft in flight and these instances are rarely, if ever, fatal.''