The worst air crash in Greek and Cypriot aviation history was caused by a single switch out of place, accident investigators concluded today.

A report by Greece’s National Aviation Safety Board found that the cabin pressure controls on a Helios Airways flight from Cyprus to Prague on August 14 last year were set to manual rather than automatic by ground staff — an error that was not picked up by the pilot or co-pilot in three subsequent checks.

The two pilots then failed to correct the switch as warnings sounded and oxygen masks fell into the passenger cabin.

Rendered unconscious by the lack of oxygen, the pilots failed to respond to radio calls as the Boeing 737-300 drifted for more than two hours through Greek airspace before running out fuel and crashing into a hillside 45km (28 miles) north of Athens. All 121 people on board, most of them Cypriots, were killed.

Today's report has been eagerly anticipated by aviation safety experts because of the mysterious circumstances of last year's crash.

As the Helios Airways jet, flying on autopilot, continued to circle in a holding pattern above Greece, two F16 fighters were scrambled to investigate the aircraft.

The two military pilots reported seeing the co-pilot slumped in his seat, unconscious passengers and one man, apparently not the Helios captain, struggling at the controls.

Today's report reveals that the F16s accompanied the stricken airliner for 30 minutes, and that one flew in close alongside, when it spotted a "person not wearing an oxygen mask entering the cockpit and occupying the Captain's seat" at 11:49am.

The F16 then spent the next five minutes trying to attract the attention of the man, who is thought to have been Andreas Prodromou, a 25-year-old qualified pilot who was working as an air steward, without success. As the aircraft's two engines flamed out from lack of fuel, the man sent out two mayday messages. The jet crashed at 12:03pm.

The report, which was prepared by Akrivos Tsolakis, head of Greece’s National Aviation Safety Board, and delivered to the country's Transport Minister, listed three "direct" causes of last year's crash, the first being: "Non-recognition that the cabin pressurisation mode selector was in the MAN (manual) position during the performance of the Preflight procedure, the Before Start checklist and the After Takeoff checklist."

According to David Learmount, the Operations and Safety Editor of Flight International magazine who has been briefed on the final report, the switch had been set to manual because ground engineers were testing the airliner's seals after a reported leak.

"It would have been ideal if they had set it back to its normal position, which is 'auto'. But they are not required to do that," he said.

Investigators blamed the pilots for failing to recognise the "Cabin Altitude Warning Horn", which would have sounded at 10,000ft, and the reasons for the deployment of passenger oxygen masks, at 18,200ft, and a third "Master Caution".

Mr Learmount, a former RAF pilot, explained that as cabin pressure fell, two dials and a set of warning lights above the pilots' heads would have informed them of the problem.

"Basically they didn't look at it," he said of the control panel. "They can't have done."

During their ascent to 34,000ft from Larnaca airport, the pilots contacted Helios Airways's operations centre to report a problem with the aircraft's cooling system. They talked for eight minutes but stopped when the aircraft passed 28,900ft. "Thereafter there was no response to radio calls to the aircraft," today's report said.

The ultimate cause of the crash was "incapacitation of the flight crew due to hypoxia (lack of oxygen)". Post-mortem examinations carried out after accident showed that many of the passengers were alive when they hit the ground.

As a result of the crash, five recommendations were made to Boeing, the aircraft manufacturer, and the US National Transportation Safety Board, for failing to improve the Boeing 737 after other sudden losses of cabin pressure. Two further recommendations were made to the Greek and Cypriot aviation authorities.

Helios Airways, which was renamed Ajet last year after the crash, was criticised for "deficiencies in the organisation, quality management and safety culture".

The budget airline narrowly won approval to keep flying in the EU last month, but has been banned from flying in foggy conditions after authorities could not guarantee safety in such conditions.

After the crash last year, the airline admitted that the same Boeing 737 had suffered a sudden decompression nine months earlier and had been forced to make an emergency landing at Larnaca.

The company said today that it had not yet seen the final report into the accident but was "deeply dismayed" that more emphasis had not been placed on the actions of the ground engineers who had moved the cabin pressure control from its normal position.

"The suggestions that the operation of Helios in any way contributed to the accident are not based on any logical analysis of the accident and have been challenged vigorously by the company," it said in a statement.

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