A Singapore Airlines flight that made an emergency landing at Suvarnabhumi Airport on Tuesday had fallen into an air pocket before it encountered turbulence, with one man killed and seven people critically injured on board, a senior airport official said.

A 73-year-old British man died during the incident, likely due to a heart attack, while head injuries were sustained among seven people critically injured, said Suvarnabhumi's general manager. One crew member was admitted to hospital.

The plane “looks okay from the outside but inside it’s a mess”, he said.

Flight SQ321 was en route from London to Singapore when it encountered “sudden extreme turbulence over the Irrawaddy Basin at 37,000 feet about 10 hours after departure”, Singapore Airlines said.

“The pilot declared a medical emergency and diverted the aircraft to Bangkok, and landed at 1545hrs local time.

“We can confirm that there are injuries and one fatality on board the Boeing 777-300ER. There were a total of 211 passengers and 18 crew on board.”

In an update posted at 7.50pm the carrier said: “18 individuals have been hospitalised. Another 12 are being treated in hospitals.”

More than 10 ambulances were mobilised to help transport injured passengers from the airport to Samitivej Hospital, according to local media reports.

Data appears to show that about 11 hours after it departed London, the plane fell from 37,000 feet to roughly 31,000 feet in only a few minutes.

With around three hours left on the journey from London to Singapore, Malaysian passenger Dzafran Azmir felt that the plane was tilting upwards and beginning to shake.

He braced himself and checked he had his seatbelt on. He did. Many of the other passengers did not, he told Reuters.

“Suddenly there was a very dramatic drop so everyone seated and not wearing a seatbelt was launched immediately into the ceiling, some people hit their heads on the baggage cabins overhead and dented it, they hit the places where lights and masks are and broke straight through it,” Azmir said.

“People dropped to the ground, my phone flew out of my hand and went a couple of aisles to the side, people’s shoes flung about,” he added.

“The crew and people inside lavatories were hurt the most because we discovered people just on the ground not able to get up. There were a lot of spinal and head injuries,” Azmir said.

The captain informed passengers they would be making an emergency landing in Bangkok.