The aircraft manufacturer has been working on a software update since the Lion Air crash last October, but the US Federal Aviation Administration (FAA) says testing and application could take months.

While Boeing insists the 737 MAX is safe, the entire fleet has now been grounded, it says, "out of an abundance caution".

On Sunday (March 10), Ethiopian Airlines flight ET 302 crashed shortly after taking off from Addis Ababa. All 159 people on board, including nine British nationals and one Irish national, were killed. Ethiopian immediately grounded its 737 MAX.

It came just five months after Lion Air flight JT 610 nosedived into the Java Sea off the coast of Indonesia minutes after taking off from Jakarta. All 189 people on board died.

The Lion Air crash was the first major catastrophe involving a 737 MAX 8 aircraft. Initial investigations suggest a new anti-stall system forced the aircraft’s nose down with the pilots unable to override it.

There has been no suggestion at this stage a similar fate befell Ethiopian Airlines flight ET 302.

However, the FAA said this week fresh evidence and “newly refined satellite data” had informed both its and Boeing’s decision to ground the 737 MAX, a decision that came after the aircraft had been taken out of service across much of the rest of the world.

Boeing is so far understood to have delivered 371 of its 737 MAX aircraft, the largest proportion serving domestic routes in the US and Canada.

A further 4,700 remain on order, including 135 from Ryanair, with option to extend this even further.

Discussing the new evidence, Dan Elwell, the FAA’s acting administrator, said on Wednesday (March 13): "It became clear to all parties the track of the Ethiopian Airlines flight was very close and behaved very similarly to the Lion Air flight."

He added the evidence found on the ground in Ethiopia “made it even more likely the flight path was very close” to that of Lion Air flight LT 610.

The FAA has now said the 737 MAX will remain grounded until at least May.

Rick Larsen, chairman of the US house transportation committee’s aviation sub-committee, has said Boeing’s software update will take several weeks to complete, with installation taking place "at least through April", the BBC reports.

The FAA, meanwhile, has said Boeing’s software fix it has been working on since the Lion Air crash might yet still take months to complete.

The CAA grounded the 737 MAX on Tuesday, a move followed by the European Aviation Safety Agency later that same day.

In Europe, Tui operates 15 737 MAX, Norwegian 18 and Icelandair three. All three airlines have pledged to continue operations with minimal disruption using alternative aircraft.

Norwegian boss Bjorn Kjos has pledged to pursue Boeing for the cost of any disruption to the airline’s operations arising from the 737 MAX being grounded.