According to the Financial Times, Easa – in a letter to Boeing’s senior management team – has flagged five issues the US aircraft manufacturer must resolve before it can take to the skies over Europe again.

The additional scrutiny could result in the aircraft returning to service in Europe later than in the US, where it is not expected to resume operations until September at the earliest.

At least one US airline, Southwest Airlines, has already pushed its 737 Max schedule back to the start of October, a month later than previously planned.

Boeing’s 737 Max was grounded worldwide following two fatal crashes in just five months.

Last October, Lion Air flight 610 came down in the Java Sea shortly after taking off from Jakarta, killing all 189 people on board.

Then in March, Ethiopian Airlines flight 302 crashed en route to Nairobi. All 157 people onboard died.

Easa ordered all airlines operating in Europe to ground their 737 Max aircraft two days after the Ethiopian Airlines tragedy on 10 March.

Preliminary investigations into both crashes suggest the same deep-seated control system was active when the aircraft came down.

Boeing is working on a software fix, as well as additional safety systems and new pilot training.

These are yet to be submitted for the US Federal Aviation Administration for approval.

Easa’s letter, the Financial Times reports, is jointly addressed to the US Federal Aviation Administration (FAA) and sets out five key changes it will require Boeing to make before it is happy to allow the 737 Max to resume operations.

These changes or assurances include Boeing resolving a new software issues with the aircraft’s flight control system, identified during simulator training; how the aircraft’s autopilot system is engaged and disengaged; and whether an average pilot actually has the physical strength to turn the cockpit trim wheel manually.

The system believed to have been involved in both the Lion Air and Ethiopian Airlines incidents was the Maneuvering Characteristics Augmentation System (Macs), which is designed to automatically determine the correct angle of the aircraft’s nose.

A faulty sensor though is believed to have passed the Mcas an incorrect reading, forcing it into a nosedive from which the pilots were unable to recover.

A number of European airlines operate the 737 Max, including Norwegian and Tui.