More than 140 travellers and crew members who were onboard a flight hit by severe turbulence landed in Singapore on a relief flight earlier this morning.

Passengers onboard SQ321 which was heading from London to Singapore, recounted scenes of "absolute terror", with one passenger saying he saw a woman with an "awful gash on her head", and heard another "screaming in agony".

A 73-year-old British man, Geoff Kitchen, died from a suspected heart attack onboard, while several others remain seriously injured.

Mr Kitchen is believed to have suffered a heart attack when the plane was hit by the turbulence. Reports say he was on his way to Singapore to start a six-week holiday along with his wife who was also on board.

The Singapore-bound Boeing 777-300ER diverted to Bangkok following the mid-air incident, making an emergency landing at 15:45 local time (08:45 GMT) with some 211 passengers and 18 crew aboard.

Smitivej Hospital, in Bangkok, said 104 people were treated and 58 remain in hospital, 20 of whom are in the intensive care unit.

There are 15 Britons still being treated in hospital, with six in intensive care, the hospital said.

Andrew Davies, a British passenger onboard the Boeing 777-300ER, told the BBC's Radio 5 Live that the plane "suddenly dropped... [with] very little warning".

"The thing I remember the most is seeing objects and things flying through the air. I was covered in coffee. It was incredibly severe turbulence," he said.

Another passenger said those not wearing seatbelts were "launched immediately into the ceiling".

"Very suddenly there was a very dramatic drop, so everyone seated and not wearing a seatbelt was launched immediately into the ceiling," 28-year-old student Dzafran Azmir told Reuters.

"I saw people from across the aisle just going completely horizontal, hitting the ceiling and landing back down in really awkward positions. People getting massive gashes in the head concussions."

Mr Azmir added that people's heads had slammed into the overhead panels above the seats and "pushed through" some of the panels.

Another Briton, Jerry, 68, was travelling to Australia for his son's wedding. He said there was no warning before the "plane plunged".

"I hit my head on the ceiling, my wife did - some poor people who were walking around ended up doing somersaults," he recalled.

“My son was thrown down on the floor two rows behind me. I heard that there was a guy hitting the roof in the toilet and he was injured quite badly, too,” he said, speaking from a Thai hospital. He added that he and his family were “lucky enough” that none of them had died.

The family had been travelling to Australia for his son's wedding on Friday, but now would not be able to make it, he said.

One Singaporean man, whose son was onboard the plane, said he was "thrown all over the place".

Chiew says his 22-year-old son was in London on holiday visiting his girlfriend, who was studying there on an exchange programme. The pair were flying back to Singapore when turbulence hit.

He told the BBC: "My son was on his way to the restroom, while his girlfriend was seated. Both are okay.

"He said he wasn't injured, he was all right - but he's a bit bruised, he was thrown all over the place."

He said his son had messaged him yesterday afternoon to say he had landed in Bangkok after the flight was diverted.

An airline official said that about 10 hours into its flight, the plane had encountered "sudden extreme turbulence" over Myanmar's Irrawaddy Basin at 37,000 feet.

Singapore Airlines also provided details on the nationalities of those on the flight, which included 56 people from Australia and 47 from the UK.

Allison Barker said she received a message from her son, Josh, who was on the plane en route to Bali: "I don't want to scare you, but I'm on a crazy flight. The plane is making an emergency landing... I love you all."

After that message, she waited for a "petrifying" two hours before hearing from him again.

"One minute, he was just sitting down wearing a seatbelt, the next minute, he must have blacked out because he found himself on the floor with other people," she told the BBC.

Josh, she said, sustained minor injuries - but she is concerned that coming close to death could have a lasting impact on him.

The head of Singapore Airlines, Goh Choon Phong, apologised on Wednesday morning, saying the carrier was "very sorry for the traumatic experience".

In a video statement, Mr Goh said the airline was "fully cooperating with relevant authorities on the investigations".

He also expressed his condolences to the family of the victim, adding that they would "render all possible assistance" to affected passengers and crew members.

Singapore Prime Minister Lawrence Wong also sent his "deepest condolences" to the family and loved ones of the deceased, adding that Singapore was "working closely with Thai authorities".

He also said Singapore's Transport Safety Investigation Bureau would conduct a thorough investigation into the incident.

Accidents involving Singapore Airlines are rare, with the carrier consistently ranking among the world's safest.

The last fatal accident occurred in 2000, when a Boeing 747 crashed while attempting to take off from the wrong runway at a Taiwan airport.

Some 83 people of the 179 people onboard were killed.

Turbulence is most commonly caused by aircraft flying through cloud, but there is also "clear air" turbulence which is not visible on a jet's weather radar.

“Injuries from severe turbulence are relatively rare in the context of millions of flights operated," aviation expert John Strickland told the BBC.

Aviation journalist Sally Gethin said wearing a seatbelt could be the "difference between life and death", explaining that anything not bolted down is at risk during severe turbulence.

Research has shown that climate change will make severe turbulence more likely in the future.