A 73-year-old Briton died and dozens of people were injured when a plane hit “sudden extreme turbulence” about 10 hours after a flight from London to Singapore took off, officials said on Tuesday.

The plane, a Singapore Airlines Boeing 777-300ER, was diverted to Bangkok, the airline said in a statement, and landed at 3:45 p.m. local time on Tuesday.

A total of 71 people, including passengers and crew members, were injured, a hospital in Bangkok said. Samitivej Srinakarin Hospital said that of the 71 passengers being treated in its medical network, six were seriously injured. Four of the injured were from Great Britain, three from Malaysia, two from New Zealand and one each from Ireland, Spain and the United States. The nationality of the other victims was not known.

The remaining passengers and crew members were examined and treated at Suvarnabhumi Airport in Bangkok, the airline said. Singapore's Ministry of Transport said it was investigating the incident and would send investigators to Bangkok, and the United States National Transportation Safety Board said it would send a representative and four technical advisers to assist. Thai Prime Minister said his government would “help with whatever is necessary for the injured.”

The man died on board the flight, said Kittipong Kittikachorn, the director of Bangkok airport, without naming him or giving the cause of death. The man's wife was injured and taken to hospital, Mr Kittipong said.

The average flight time on this route is just under 13 hours. Mr Kittipong told a press conference that he boarded the plane after landing and described the scene as “chaos”.

Flight SQ321 departed Heathrow at 10:38 p.m. local time on Monday with 211 passengers and 18 crew members. The airline said it encountered turbulence at an altitude of 37,000 feet over the Irrawaddy Basin over Myanmar. Data on Flight Radar 24, a website that compiles public information about flights, appears to show that the flight climbed from 37,000 feet to an altitude of about 31,000 feet in just a few minutes about 10 hours later.

The pilot declared a medical emergency and diverted the flight to Bangkok.

About 100 uninjured passengers were expected to be flown to Singapore on Tuesday, Mr Kittipong said, describing them as being in a state of shock.

It is unclear what caused the episode. As the plane flew over Myanmar, satellite data showed a strong storm began to form and bubble into higher altitudes, suggesting the atmosphere in the region was becoming unstable. The plane was also heading toward other storms developing along Myanmar's coast.

Deaths due to turbulence are rare.

The last time a passenger died from a turbulence-related injury on a commercial American airline flight was in 1997, when a United Airlines flight from Tokyo to Honolulu experienced severe turbulence over the Pacific Ocean, according to an NTSB report investigation shows. According to the investigation, the passenger was not wearing a seatbelt and flew out of her seat, possibly hitting her head on the luggage rack.

Current research suggests that turbulence is increasing and climate change is a cause. In particular, this involves increased carbon dioxide emissions, which affect air flows.

José Alvarado, a pilot for Icelandic airline Play, said that in his experience, clear-air turbulence, which is most common at high altitudes, can occur without warning. That's why he asks passengers: “Even if there is no turbulence, you should buckle up.”

Once, more than two decades ago, while working as a flight attendant, he experienced turbulence on a flight from Madrid to Buenos Aires so severe that he was thrown upward when the plane suddenly plummeted about 4,000 feet. His back and shoulder blades hit the ceiling of the cabin before he was thrown back down.

“I was just jumping up and down,” he said, adding that some passengers were also injured. He said he hasn't experienced anything like it since.

Chee Hong Tat, Singapore's transport minister, said in a statement that he was “deeply saddened to learn of the incident.”

Singapore Airlines expressed its condolences to the family of the person who died on the flight, adding: “We deeply apologize for the traumatic experience our passengers and crew members endured on this flight.”

Reporting was contributed by Judson Jones, Jenny Gross, Christine Chung And Mark Walker.