Europe's safety regulator has ordered urgent inspections on almost a third of the Airbus A380 fleet after cracks were found inside the wings of the world's largest jetliner.

No superjumbos have been grounded but the most heavily used aircraft -- those subjected to at least 1,800 take-offs and landings that impose the most strain on an airframe -- must be examined within four days, authorities said.

The European Aviation Safety Agency acted after European planemaker Airbus disclosed two sets of cracks on its A380s just two weeks apart, and barely four years after the 525-seat double-decker passenger jet entered service.

The second type of cracks, which like the first appeared on a bracket joining the exterior to metal ribs inside the wings, was 'more significant' and could develop on other aircraft if the problem is not addressed, the Cologne-based agency said.

'This condition, if not detected and corrected, could potentially affect the structural integrity of the aeroplane,' EASA said in an airworthiness directive issued on Friday.

Inspectors ordered checks within six weeks for another category of aircraft with a history of 1,300 to 1,800 cycles.

In total, the two-speed recommendations cover 20 aircraft  including 10 from Singapore Airlines, seven from Emirates, one from Air France and two test planes, according to a published list of serial numbers at a380production.com.

Experts say engineers must carry out visual checks by climbing inside the ribcage of each UK-built wing. The process is expected to take each jet out of service for about 24 hours.

Airbus said the order demonstrated the 'airworthiness process is working' and reiterated the aircraft is safe to fly. It has dismissed calls to ground the jets but the Australian engineering union which handles routine servicing and engine checks on superjumbos operated by Qantas Airways said it was concerned about the speed of the planemaker's response.

'They have described these as tiny cracks, but every crack starts off as a tiny crack and they can grow very quickly,' said Stephen Purvinas, federal secretary of the Australian Licensed Aircraft Engineers Association who has called for the grounding.

The discovery comes 14 months after a Rolls-Royce  engine blowout on a Qantas A380 triggered global headlines. It was during a $130 million repair job on that same aircraft that hairline cracks in the wing first came to light.

Industry sources said the second set of cracks appeared on aircraft operated by Dubai's Emirates, the largest A380 customer. Emirates declined comment but said on Thursday it continued to monitor its fleet and safety was its top priority.

Air France and Singapore Airlines confirmed they would carry out checks and stressed their commitment to passenger safety.  Other operators include Korean Air, China Southern and Germany's Lufthansa.  - Reuters