A spokesperson for the Samitivej Srinakarin Hospital in Bangkok, Thailand, said six Britons are among 20 people still in intensive care after the Boeing 777 Singapore Airlines flight sharply descended 6,000ft (1,800 metres) after hitting the turbulence over the Andaman Sea.

Others in the ICU include six Malaysians, three Australians, two Singaporeans and one person each from Hong Kong, New Zealand, and the Philippines.

Some of the 211 passengers on board flight SQ321 described their "sheer terror" as the aircraft began shuddering, causing people to be "launched into the ceiling" while others laid paralysed on the floor.

Geoff Kitchen, 73, from Gloucestershire in the UK died from a suspected heart attack after the freak incident despite flight crew trying to revive him for 20 minutes.

One of the passengers, Josh Silverstone, told Sky News that there was an alert for the seatbelts to come on and then there was a "huge bang".

"I remember waking up on the floor and just hearing people crying and I am looking around and seeing blood and the ceiling falling through," he said.

Mr Silverstone said there was an elderly lady in front of him who couldn't move and couldn't remember her name or why she was on the flight, while cabin crew members, who had their own injuries, were lying on the floor with passengers making sure they were okay.

The Samitivej Srinakarin Hospital said it had provided medical care to a total of 104 people after the flight was diverted to Thailand.

Read more:
Images show damage inside plane
Is flight turbulence getting worse?

It remains unclear what exactly caused the turbulence, but data from Flightradar24, said "the flight encountered a rapid change in vertical rate, consistent with a sudden turbulence event".

At 3.03pm local time, the flight changed course and began its diversion to Bangkok and around 20 minutes later declared an emergency.

Tim Atkinson, an aviation consultant and pilot, told the Sky News Daily podcast he believes "it's fairly clear" the Singapore Airlines flight "encountered atmospheric turbulence".

He noted that the area - called the Intertropical Convergence Zone - where the Boeing 777 plunged 6,000 feet is "renowned among pilots, and I dare say passengers, for turbulence".