Boeing's 737 Max plane is safe to return to service in the UK and the European Union, regulators have said.
It ends a 22-month flight ban for the jet, which followed two crashes which caused 346 deaths.
The plane had already been cleared to resume flying in North America and Brazil.
But this week a senior manager at Boeing's 737 plant in Seattle warned that recertification had happened too quickly.
Regulators in the US and Europe insist their reviews have been thorough, and that the 737 Max aircraft is now safe.
The European Union Aviation Safety Agency (Easa), which regulates aviation in 31 mainly EU countries, said it now had "every confidence" in the plane following an independent review.
"But we will continue to monitor 737 Max operations closely as the aircraft resumes service," said executive director Patrick Ky.
"In parallel, and at our insistence, Boeing has also committed to work to enhance the aircraft still further in the medium term, in order to reach an even higher level of safety."
The UK Civil Aviation Authority (CAA), which oversees UK aviation now Britain has left the EU, said the work to return the 737 Max to the skies had been "the most extensive project of this kind".
It said it was in close contact with Tui, currently the only UK operator of the aircraft, as it returned the plane to service.
"As part of this we will have full oversight of the airline's plans including its pilot training programmes and implementation of the required aircraft modifications."
The 737 Max's first accident occurred in October 2018, when a Lion Air jet came down in the sea off Indonesia.
The second involved an Ethiopian Airlines version that crashed shortly after takeoff from Addis Ababa, just four months later.
Both have been attributed to flawed flight control software, which became active at the wrong time and prompted the aircraft to go into a catastrophic dive.
Easa said it had done a full investigation independent of Boeing or the US Federal Aviation Administration and "without any economic or political pressure".
As a result, it demanded software upgrades, electrical working rework, maintenance checks, operations manual updates and crew training.
"We asked difficult questions until we got answers and pushed for solutions which satisfied our exacting safety requirements," Mr Ky said.
The CAA said it had based its decision on information from Easa, the US Federal Aviation Agency and Boeing, as well as "extensive engagement" with airline operators and pilots.
It comes days after a report by Ed Pierson, a former Boeing manager, claimed that regulators and investigators had largely ignored factors that may have played a direct role in the accidents.
Mr Pierson said that further investigation of electrical issues and production quality problems at the 737 factory in Seattle was badly needed.
On Wednesday Naoise Connolly Ryan, whose husband Mick died in the Ethiopian Airlines crash, said that the families of victims "still do not have a full accounting of what happened and why".
"Ultimately we are more determined than ever to find out exactly what Boeing knew about this dangerous aircraft, and hold them accountable for the deaths of our loved ones."
Boeing has already agreed to pay $2.5bn (£1.8bn) to settle US criminal charges that it hid information from safety officials about the design of the planes.
The US Justice Department said the firm chose "profit over candour", impeding oversight of the planes.
About $500m of that will go to families of the people killed in the tragedies.
However, attorneys for the victims of the Ethiopian Airlines crash have said the deal would not end their pending civil lawsuit against Boeing.
On Wednesday, Boeing posted a record $12bn annual loss after it delayed its all-new 777X jet for the third time, incurring huge charges.
The coronavirus crisis has caused demand for the industry's largest jetliners to fall, with airline customers shunning deliveries of planes due international travel restrictions.
The 737 Max has already been cleared to fly in North America and Brazil - now it has the go-ahead from European regulators as well.
It's a major step for Boeing - although with the current travel restrictions in place, it's likely to be a while before the decision has much practical effect.
But the controversy won't end there. Relatives of those who died in the Ethiopian Airlines accident have made it clear they haven't heard enough to be sure the aircraft - modified in accordance with regulators' wishes - is truly safe.
And this week, a former senior manager at the 737 factory told the BBC why he thought existing planes might still be carrying potentially dangerous manufacturing defects.
That may explain why Easa has also chosen to publish a report setting out the detailed reasoning behind its decision.
Ultimately, the 737 Max may we'll have decades of successful service ahead of it. But for the moment, winning back passenger confidence will be a formidable challenge.