Boeing 737 MAX jets may not return to commercial service until next year, due to needed software updates for the flight-control system and time needed to prepare the grounded planes for flight.

Boeing and airlines flying the MAX had hoped to return the planes to service this year, but continued disagreements about the extent of needed upgrades and time needed to secure regulatory approval appear to make a quick return unlikely.

In a statement issued on Monday, Boeing said: "We deeply regret the impact the 737 MAX grounding is having on our customers and their passengers. Boeing is working very closely with the FAA on the process they have laid out to certify the 737 MAX software update and safely return the MAX to service. We will submit the final software package to the FAA once we have satisfied all their certification requirements. We will not comment on media speculation on that schedule."

Boeing "was not transparent or accountable in the self-certification process for the plane," James Hall, managing partner of Hall & Associates in Washington and former chairman of the National Transportation Safety Board, told China Daily.

"At present, it's hard to see an end to this story," he said. "There are congressional hearings, civil and criminal investigations, and lawsuits from family members who lost loved ones. This has damaged the reputations of Boeing and the FAA around the world."

Major airlines have pulled the MAX jets from scheduled flights until November. Analysts estimate it will take at least 45 days to return the grounded aircraft to commercial service after securing regulatory approval, pushing the first commercial flights into next year.

The Fraud Division of the US Justice Department has begun a criminal investigation into the development and certification of the MAX jet. The FAA routinely delegates much of its aircraft certification to approved manufacturers through a congressionally authorized program. Hall and other critics have argued that is a mistake and have called for the regulations to be rewritten.

Testifying before the Senate Appropriations subcommittee on the FAA's 2020 budget, Secretary of Transportation Elaine Chao said: "The FAA does not build planes. They certify. But this method of having the manufacturer also be involved in looking at these standards is really necessary because once again the FAA cannot do it on their own."

Boeing MAX jets were grounded worldwide following crashes March 10 in Ethiopia and Oct 29, 2018, in Indonesia that killed a total of 346 passengers and crew. Preliminary investigations suggest the aircraft's Maneuvering Characteristics Augmentation System (MCAS), an automated anti-stall device, forced the noses of Lion Air and Ethiopian Airlines flights down and into a fatal plunge when it erroneously determined the planes were about to stall. To avoid a stall, MCAS points the nose of the plane down to gain air speed.

American Airlines, which has 24 MAX jets, plans to keep the aircraft off its schedule until Nov 3, two months longer than initially planned. United Airlines, which has 14 MAXs, extended flight cancellations of the plane until Nov 3, a month longer than expected. Southwest Airlines, which has 34 MAX jets, has canceled about 150 flights a day.

"American Airlines remains confident that impending software updates to the Boeing 737 MAX, along with the new training elements Boeing is developing in coordination with our union partners, will lead to recertification of the aircraft this year," the airline said in a statement.

China was the first nation to ground MAX jets following the fatal crashes.