A KLM plane had to be diverted to Heathrow after a passenger was taken ill and then died on board.
Police were treating the Boeing 777 as a crime scene on Monday night after the Amsterdam-bound plane from Lima was diverted after the medical emergency.
The plane landed at around 2.15pm, and was met by paramedics who declared the man, thought to be in his 30s or 40s, dead at the scene.
John Hopwood, a nurse from Manchester who was returning after a holiday in South America, was sat six rows behind the deceased passenger. He said passengers were kept on board for five hours before being told the plane would be treated as a crime scene and they had to disembark.
A senior police officer then told them in one of Heathrow's terminals that the death was being treated as "suspicious".
Mr Hopwood told the Guardian: "There was no commotion on board. We were just told initially that someone had taken ill.
"The police officer said they were bringing a forensics team over from the other side of London.
"We did ask why it took so long for it to be designated as a crime scene. We were kept on the plane for five hours and during that time people would have been walking past the seat where the passenger was."
A spokesperson for the Metropolitan police confirmed that an investigation was under way into the circumstances of the death, which was being treated as unexplained.
Back in June, a Delta Airlines flight from Detroit to Phoenix made an unexpected landing in Colorado Springs after a woman became "unresponsive" on on board - and was later pronounced dead.
Flight attendants and a doctor on board performed CPR on the woman in a bid to revive her but, after the plane landed, officials determined that the passenger was dead.
Woman sits next to corpse on long-haul flight after fellow passenger dies
Delta flight diverted after 'woman dies on plane'