Ryanair, Norwegian and Tui are among dozens of airlines that fly the 737 MAX, the Boeing aircraft model involved in a fatal crash in Ethiopia on Sunday that left 157 crew and passengers dead.
It was the same type of plane that plunged into the Java Sea in Indonesia last October in similar circumstances, raising questions over the airworthiness of a jet that has only been in service since 2017.
On Monday, China's aviation authority ordered the grounding of nearly 100 737 MAX aircraft belonging to 13 airlines "in line with the management principle of zero tolerance for safety hazards and strict control of safety risks", followed by authorities in Indonesia. Ethiopian Airlines, which flies direct to London Heathrow from Addis Ababa, and Cayman Airlines have, too, grounded their 737 MAX planes.
What is the aircraft and who flies it?
The 737 MAX is an updated version of the Boeing 737, the best-selling commercial jet in history, but designed to be quieter and more fuel efficient than its predecessor.
More than 5,000 of the plane have been ordered around the world making it the fastest selling aircraft in Boeing history, with 350 currently in service with carriers.
It has been pegged as the short-haul jet of the future. Ryanair, which is awaiting the first delivery from an order of 135, has said the plane is "game-changing".
The UK Civil Aviation Authority (CAA) said there are five 737 MAX 8 aircraft registered in the UK, with a sixth due to enter service later this week; it did not say with which airline. It said it was liaising "very closely" with the European Union Aviation Safety Agency (EASA) "as the facts of this incident are established".
A spokesperson for Norwegian Air Shuttle, which has 18 of the aircraft and another 110 on order, said the airline remains "in close dialogue with Boeing", adding that in the wake of the Lion Air crash, new training routines were introduced to its 737 MAX manuals.
A spokesperson for Tui, which has 11 of the aircraft and another 72 on order, said the holiday operator would not comment on speculation but that it was "as always, in close contact with the manufacturer", adding: "We have no indication that we can't operate our 737 MAX in a safe way like we do with all other planes in our network."
Ryanair is yet to respond to a request for comment.
Virgin Australia says it is too early to comment on whether its order of Boeing 737 MAX 8s will change. The airline has ordered 30 of the Boeing 737 MAX 8s and 10 Boeing 737 MAX 10s and the first of the new aircraft are due to be delivered at the end of the year.
Other popular airlines to order the plane include Southwest, United and American in the US, Flydubai, Turkish Airlines, Air Canada and Icelandair.
Is the plane safe?
At the centre of the debate into the safety of the 737 MAX is a system adaptation designed to protect the jet from stalling by forcing the nose of the plane down in certain circumstances. There were concerns that pilots had not been sufficiently informed of the new feature.
In the wake of the October crash the Federal Aviation Administration (FAA) issued an emergency directive concerning erroneous readings from an "angle of attack" sensor that might lead to "repeated nose-down trim commands".
"This condition, if not addressed, could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain," the airworthiness directive read.
Saj Ahmad, chief analyst at StrategicAero Research, said however it is premature to say there is a safety issue.
"It's crucial at this stage to look at the crash in total isolation and not to compare it in anyway to the Lion Crash last year," he said. "Unless there is conclusive evidence, it would be irresponsible to draw any conclusions or parallels."
He said Boeing, the FAA and other regulatory agencies would now pore over evidence to understand what caused the Ethiopian Airlines crash. He said the carrier is "a very well oiled machines with a very high standard of safety adherence, as well as operating a very modern and young fleet of advanced airplanes like the 737 MAX".
"It should be noted that airlines like American, United, Norwegian, Silk Air, flydubai, WestJet, Air Canada, Jet Airways, SpiceJet and countless others are flying the MAX family today with no issues at all," he said.
"While Ethiopian Airlines, Cayman Airlines and Chinese carriers have voluntarily stood down their MAX fleets, that doesn't signify fault - it signifies prudence and wanting to better understand what's happened and if immediate corrective measures are needed to further enhance the robustness and safety of the 737 MAX."
Boeing has said it is sending a technical assistance team to the site of the crash. It said it is postponing the "external debut" of its 777X model and related media events scheduled for this week because of the accident. There is no change to the plane's schedule or progress, Boeing said.
Why is the plane so popular?
The 737 MAX is an update to Boeing's trusted 737 model, which has long been a workhorse for short-haul airlines. The new version - which comes in four variants (7, 8, 9 and 10, according to size and capacity) - will offer airlines greater fuel efficiency and range and passengers added comfort.
Boeing says of the MAX range: "The 737 MAX family is designed to offer the greatest flexibility, reliability and efficiency in the single-aisle market.
"Every airplane will feature the new Boeing Sky Interior, highlighted by modern sculpted sidewalls and window reveals, LED lighting that enhances the sense of spaciousness and larger pivoting overhead storage bins."
Who has ordered the Boeing 737 MAX?
As of September 30, 2018
VietJet : 100
Capital Group: 100
American Airlines: 100
Norwegian Air Shuttle: 110
Jet Airways: 125
United Airlines: 135
Gol Transportes Aereos: 135
Air Lease Corportation: 168
Lion Air: 201
Southwest Airlines: 280
The Telegraph, London