The manufacturer has previously insisted its computer-based training was adequate.

However, the new guidance comes amid unprecedented scrutiny on the manufacturer after two fatal crashes involving the 737 Max, killing 346 people. The aircraft has been grounded since the second in March last year.

Boeing said its new recommendation was based on changes made to the aircraft following the crashes and "test results".

It previously said pilots able to fly older 737s only needed a short computer course of about an hour to adapt to the 737 Max’s controls, avoiding extra training costs for airlines.

The US Federal Aviation Administration (FAA) last year OKed Boeing’s stance in a technical advisory, but this has come under more intense scrutiny.

In a statement, Boeing said: "Boeing is recommending 737 Max simulator training in addition to computer based training for all Max pilots prior to return to service of the 737 Max.

"This recommendation takes into account our unstinting commitment to the safe return of service, as well as changes to the airplane and test results. Final determination will be established by the regulators."

Boeing’s interim chief executive Greg Smith, who has stepped in after predecessor Dennis Muilenburg was ousted shortly before Christmas after overseeing the Max crisis, added: “Safety is Boeing’s top priority. Public, customer and stakeholder confidence in the 737 Max is critically important to us.

"With that focus, Boeing has decided to recommend Max simulator training, combined with computer-based training, for all pilots prior to returning the Max safely to service."