A pathologist has told an inquest that a British couple who suddenly fell ill after staying in an Egyptian hotel room died of carbon monoxide poisoning.
But experts could not be sure of the source of the poisonous gas that killed John Cooper, 69, and his wife Susan, 63, Blackburn Coroner's Court heard.
The couple, from Burnley, Lancashire, died suddenly on 21 August 2018 after becoming ill.
They were staying at the Steigenberger Aqua Magic Hotel in Hurghada.
A statement from a German tourist read at the inquest said there was a bed bug infestation in the room next door to the Coopers which was treated with a pesticide, referred to as Lambda, at lunchtime, with the Coopers falling ill in the early hours and dying the next day.
The two rooms had an adjoining door, but this was kept locked.
Home Office pathologist Dr Charles Wilson gave a cause of death for Mr Cooper as carbon monoxide toxicity and heart disease, and carbon monoxide toxicity for Mrs Cooper.
Toxicology expert Professor Robert Chilcott told the hearing he detected the presence of carbon monoxide in blood samples from the bodies of the couple but he could not be certain of the levels.
He said the levels were sufficient to suggest "severe exposure" to carbon monoxide.
Prof Chilcott suggested in less developed countries the pesticide Lambda is sometimes diluted with another substance, dichloromethane, which causes the body to metabolise or ingest carbon monoxide.
He added a 10-hour exposure duration would be "sufficient to cause carbon monoxide poisoning."
He also said there were occupational limits in the UK about how much you are able to inhale and spraying it, as in the hotel room, would "rapidly exceed" those exposure limits.
Dr Nick Gent, a former senior medical adviser to Public Health England, said he agreed on the presence of carbon monoxide in the Coopers' blood but had no idea about the source.
Earlier Kelly Ormerod, the Coopers' daughter, described her parents as fit and healthy for their age and said they had been enjoying a "brilliant" holiday with her, their three grandchildren and family friends.
Mr Cooper, a builder, and his wife, Susan, a cashier at a bureau de change in a Thomas Cook travel agent, were found seriously ill in their room by Ms Ormerod after they failed to emerge for breakfast.
Her father was declared dead on the hotel room floor and his wife in hospital hours later.
The hearing was adjourned until Friday.