A woman has died and a man is in a coma after they were both taken ill on a transatlantic flight.
The British pair, thought to have taken drugs, were on a Thomsonfly flight from Cancun, Mexico, to Birmingham. It had to be diverted to Portugal.
A spokeswoman for a hospital in Lisbon said the 23-year-old woman was dead on arrival. A 21-year-old man was in a coma and in a critical condition.
She said the two showed signs of having "taken illegal substances".
Leonora Bettencourt, director of the Espirito Santo hospital told a Lisbon radio station that hospital tests had already identified the substance but declined to say what it was.
A Foreign Office spokesman would only confirm the pair were from Britain.
"A police investigation is under way and an autopsy will be carried out," he said.
A spokeswoman for Thomsonfly said the plane's pilot diverted the aircraft after a male passenger had a heart attack.
She said he was treated on board and once the plane had landed he left with three other passengers who were travelling with him.
The flight left the Mexico on Tuesday at 0100 (GMT) and arrived at the Azores at 0730. It left for Birmingham a couple of hours later.