Qantas has defended its maintenance and safety record after a giant hole was blown in the fuselage of one of its 747s at 29,000ft.

The Australian airline must now check all oxygen cylinders on the fleet amid the possibility that said an exploding canister may have caused the incident at 29,000ft, triggering a loss of cabin pressure during a London to Melbourne flight.

Passengers reported hearing a loud bang before the aircraft rapidly lost altitude and said the Boeing 747-400 had a hole the size of a mini-van on the right of its fuselage following an emergency landing in the Philippines, with 346 passengers and 19 crew onboard.

Neville Blyth of the Australian Transport Safety Bureau said: "I can confirm that number 4 cylinder from the forward cargo hold of the aircraft is missing and unaccounted for."

He said parts of the valve of the cylinder and its handle were found on the cabin floor of the plane or near the hole in the fuselage, but the cylinder itself was missing.

According to the airline's chief executive, Geoff Dixon, Qantas is still waiting to be given full access to the aircraft, which remains on the tarmac in Manila, but said the plane was in good condition when it left Hong Kong following a stop-over.

He said major maintenance work on the plane had been carried out in Australia and could not speculate on what caused the hole.

Mr Dixon said: "We believe everything on that aircraft was in good shape when it took off. Incidents do happen. This is a tremendously bad one, and it's one we regret."

Qantas has 225 aircraft around the world and a reputation stretching back almost 90 years as one of the safest airlines.

Mr Dixon said the company is determined to maintain its strong safety record, adding: "I'd be disappointed at anything ruining Qantas' safety record. It is an enviable record.

"The flip side to this of course is our reputation for operational excellence and training came through, in that our pilots and our cabin crew just performed tremendously under much pressure, during a very, very bad accident."

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