Boeing and Southwest Airlines have announced the carrier will continue to build its business around the 737 Max family - in what amounts to a huge endorsement of the troubled aircraft.

The low-cost carrier has placed a new order for 100 planes and 155 options across two models.

The deal comes after a multi-year fleet evaluation by Southwest and means that Boeing and its suppliers could build more than 600 new 737 Max jets for the airline through to 2031.

The plane has just returned to service after a grounding of nearly two years following fatal crashes in Indonesia and Ethiopia.

Southwest had been exploring options to modernise the largest component of its fleet: the 737-700 that serves the airline’s needs for a 140-150 seat airplane.

With the new agreement, the airline reaffirmed the 737-7 as its preferred replacement and growth plane.

The jet will complement the 737-8, which serves Southwest’s needs for a 175-seat model.

“Southwest Airlines has been operating the Boeing 737 series for nearly 50 years, and the aircraft has made significant contributions to our unparalleled success.

“Today’s commitment to the 737 Max solidifies our continued appreciation for the aircraft and confirms our plans to offer the Boeing 737 series of aircraft to our employees and customers for years to come,” said Gary Kelly, Southwest chairman and chief executive.

“We are proud to continue our tradition of being the world’s largest operator of an all-Boeing fleet.”

The new purchase agreement takes Southwest’s order book to 200 737-7s and 180 737-8s, more than 30 of which have already been delivered.

Southwest will also have 270 options for either of the two models, taking the carrier’s direct-buy commitment to more than 600 airplanes.

The airline also plans additional 737 Max jets through third-party lessors.

“Southwest Airlines has long been a leader and bellwether for the airline industry and this order is a big vote of confidence for commercial air travel.

“As vaccine distribution continues to pick-up, people are returning to the skies and fuelling hopes for a full recovery and renewed growth across our industry,” said Stan Deal, president and chief executive of Boeing Commercial Airplanes.