A passenger has been killed and multiple passengers have suffered various injuries after a Singapore Airlines (SIA) flight from London to Singapore encountered severe turbulence.

Singapore media reported that SIA flight SQ321 left London's Heathrow Airport at 2230hrs en route to Singapore, but made an emergency landing in Bangkok.

It is reported that a 73-year-old British man died during the incident, likely due to a heart attack, Suvarnabhumi airport general manager Kittipong Kittikachorn told a press conference. Seven people were critically injured with head injuries.

A total eighteen people have been hospitalised and 12 are being treated in hospitals, said SIA. A total of 211 passengers and 18 crew members were on board the Boeing 777-300ER.

An SIA team was also on the way to Bangkok to provide additional assistance, media reports stated.

"Our priority is to provide all possible assistance to all passengers and crew on board the aircraft," said SIA.

"We are working with the local authorities in Thailand to provide the necessary medical assistance, and sending a team to Bangkok to provide any additional assistance needed," SIA said.

According to Singapore's Transport Safety Investigation Bureau (TSIB), the flight encountered severe turbulence 10hrs after departure and the pilot diverted the flight to Bangkok for an emergency landing. TSIB is opening investigation into the incident, Singapore medias reported.