Qantas, once dubbed the world’s safest airline, has been rocked by another safety scare after a fault led to one of its planes plummeting 25,000ft mid-flight.

Passengers grabbed oxygen masks as the cabin became depressurized due to a faulty air conditioning system. The airline said the crew made a “rapid but controlled descent”. The Boeing 737, carrying 100 passengers six crew, landed safely at Melbourne Airport.

There were no reported injuries and Qantas said that there was no flight safety issue.

A spokesperson for the airline said: “Emergency services, I believe, were on standby but were not required. Engineers are now assessing the aircraft.

One passenger told the Herald Sun newspaper:  “All of the oxygen masks dropped down. The captain came across and said: ‘This is an emergency!’”.

Passengers were ordered to keep the oxygen masks on for about five minutes.

The incident comes after a series of emergencies that have rocked the Australian flag carrier’s reputation as one of the world’s safest airlines.

Last November it had to ground its entire fleet of six A380 superjumbos after one of the Rolls-Royce Trent 900 engines exploded in mid-air short after take-off from Singapore.

The total cost of grounding the fleet and replacing 16 of the A380 engines has been estimated to be £50million.

The day after a Qantas 747-400 aircraft had a mid-air crisis when flames burst from an engine shortly after take-off from Singapore.