Flight death is blamed on drugs.
A Thomsonfly flight from Cancun in Mexico to the city was forced to divert to Portugal's Azores Islands yesterday.
Leonora Bettencourt, director of the Santo Espirito Hospital in Angra do Heroismo, near where the emergency stop was made, told Lisbon radio station TSF that a 23-year-old woman was dead on arrival.
A 21-year-old man was in a coma, she said.
Ms Bettencourt said the two showed signs of having "taken illegal substances". She said hospital tests had already identified the substance but declined to say what it was because of a police investigation.
A Foreign Office spokesman could not confirm any personal details about the pair, but said both were from Britain.
He said the woman had died on board the flight and the man was in a "critical" condition in hospital.
He said: "Our consular officials are in contact with the hospital. Next of kin have not yet been informed."
He would not comment on speculation about a drugs overdose, adding: "A police investigation is under way and an autopsy will be carried out."
A spokeswoman for Thomsonfly said the plane's pilot had decided to divert the aircraft due to a male passenger suffering a heart attack on board.