Detectives at Heathrow Airport have begun an investigation after an Angolan man became unwell and died as he was being deported from the UK.
The 46-year-old was being escorted by three security guards on a plane preparing to leave the west London airport on Tuesday night.
After falling ill he was taken to nearby Hillingdon Hospital where he died soon after arriving.
The Prisons and Probation Ombudsman is also investigating the matter.
A Metropolitan Police spokesman said inquiries were continuing to establish the full circumstances of the incident.
There have been no arrests.
A UK Border Agency spokesman said: "We can confirm that a detainee was taken ill on a flight preparing to depart from the UK last night [Tuesday].
"Paramedics were called and he was taken to hospital where he died."