Following two fatal crashes that claimed 346 lives, the Boeing 737 Max has already been grounded for over 10 months. And now the aircraft manufacturer has conceded the plane will not be flying commercially for at least another six months.

Safety regulators around the world closed their skies to the Boeing 737 Max in mid-March 2019, following the second accident – involving Ethiopian Airlines flight ET302 from Addis Ababa to Nairobi, in which all 157 people on board died.

Like the first crash, which killed 189 on a Lion Air Boeing 737 Max departure from Jakarta in October 2018, flight control software forced the nose of the plane down.

Boeing has been working to fix the problem and convince the Federal Aviation Administration (FAA) and its counterparts around the world that the updated software is safe.

Originally the firm predicted the 737 Max would be flying in the final quarter of 2018.

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Just a week ago American Airlines said it planned to resume Max operations on 4 June 2020.

But now Boeing estimates that the “ungrounding” of the plane will not begin until “mid-2020”.

The plane manufacturer said the announcement was to help airlines plan their operations, and added: “This updated estimate is informed by our experience to date with the certification process.

“It is subject to our ongoing attempts to address known schedule risks and further developments that may arise in connection with the certification process.

“It also accounts for the rigorous scrutiny that regulatory authorities are rightly applying at every step of their review of the 737 Max's flight control system.

“We acknowledge and regret the continued difficulties that the grounding of the 737 Max has presented to our customers, our regulators, our suppliers, and the flying public.”

Once the FAA and others agree to allow flights, it will typically take one month or more before the plane is flying passengers around. There must be mandatory simulator training for pilots and test flights for the grounded planes – most of which will not have flown for well over a year.

So a return to service before August seems most unlikely. Even then airlines will be averse to removing pilots from normal operations at the busiest time of the year for training.

The biggest users of the Max – including Southwest, Air Canada and American Airlines – will be dismayed that they cannot operate their peak.

It is thought that Ryanair, which has the largest order book for the Boeing 737 Max in Europe but has not yet received any of the planes, may now defer deliveries until 2021.

The Irish carrier typically grounds some planes during the quiet winter season from October to March, and will have no need for extra capacity until operations ramp up for summer 2021.

One piece of good news for the troubled planemaker: the much-delayed maiden flight of the Boeing 777X wide-bodied jet is now expected to take place on Thursday 23 January.