However, it would have to be a purposeful decision from the flight deck and could not happened accidentally due to a medical or mechanical emergency.

Earlier, a senior commercial pilot and other aviation experts told Fairfax Media that if the flight’s pilots had become incapacitated, the passengers and cabin crew may have flown for seven hours aware that there was a problem but unable to raise the alarm.

However, other commercial airline pilots have since revealed there is a secret emergency procedure that would allow airline staff to get into the cockpit in the event that the pilots were, for example, rendered unconscious by a mechanical fault.

Fairfax Media has chosen not to detail the procedure.

But it can be by overriden by the flight deck, meaning the pilots do have the ability to block passengers and crew from contacting the outside world.

The new information means that if there had been an incident that left the pilots on MH370 physically incapacitated, cabin crew could have sent a message via the cockpit.

But if one or both of the pilots had begun to act suspiciously, the cabin crew could have been denied access and been left helpless.

Mobile phones may have been out of range, and the satellite phones which existed in business class could have been disabled, either purposefully or accidentally, by the same incident that eliminated the plane's tracking systems.

‘‘There are no communications available from the cabin to the ground ... only from the cockpit," said Professor Jason Middleton, head of University of New South Wales School of Aviation.

Professor Middleton said post 9-11 security measures meant passengers and crew were isolated from the outside world if the cockpit was inaccessible.

‘‘It’s the modern world [that says] the only way to protect against illegal activities and hijacking is for the pilots to be safely ensconced so no one can get at them and no one can get at the systems.’’

Professor Middleton said the approach usually worked but that so far flight MH370 had proved to be an "unprecedented case".

The flight path of the doomed Malaysia Airlines jet, established by British satellite company Inmarsat through the plane’s "ping" data, has shown that it flew for more than seven hours after it turned back from its scheduled flight path over the South China Sea on March 8.

Nobody yet knows why the plane deviated from its trajectory, or whether the pilots acted intentionally or were thwarted by some mechanical fault.

One theory already reported is that a catastrophic loss of pressure rendered all aboard unconscious, leaving the plane to fly on auto-pilot as a "ghost flight" until it crashed into the southern Indian Ocean.

An Australian commercial pilot, who did not want to be named, said flight MH370 would have had a reinforced cockpit door for security reasons. If the crew could not access the cockpit because they were being kept out by the pilots, they would be effectively helpless.

"There is no way they could raise the alarm," he said.

There has been much speculation as to why no phone calls or messages were received from passengers or crew on the missing flight.

American airline pilot and aviation author and blogger Patrick Smith said the lack of calls did not necessarily support the theory because unless a plane was flying low and within range of a mobile phone tower, mobile phones would not work.

Vincent Lau, an electronics professor specialising in wireless communications at Hong Kong University of Science and Technology, also told the New York Times that the altitude of the plane might have meant mobile phones could not connect with ground stations.

According to the Malaysia Airlines website, in-flight entertainment systems in business class on a Boeing 777-200, the model of flight MH370, are equipped with satellite phones.

Mr Smith said that this entertainment system could, however, be disabled as was the jet’s transponder and Aircraft Communications Addressing and Reporting System (ACARS). No one yet knows how and why these systems were not working.

Clarification: An earlier version of this story reported that the pilots could have been unconscious and left the passengers stranded, however later advice contradicted this.