A pathologist told an inquest that Thomas Cook travel agent Susan Cooper and her husband John died of carbon monoxide poisoning while staying at a hotel in Hurghada.

However, experts told Blackburn Coroner’s Court that they couldn’t be sure of the source of the poisonous gas.

Previously, the hearing had heard a statement from a German tourist who said his room next to the Cooper’s at the Aqua Magic Hotel had been treated for bedbugs the day before the couple became ill.

It was claimed the room had been treated with the pesticide Lambda the previous lunchtime.

Home Office Pathologist Dr Charles Wilson gave a cause of death for John Cooper as carbon monoxide toxicity and heart disease, and carbon monoxide toxicity for Susan, reported the BBC.

Toxicology expert Professor Robert Chilcott told the hearing there was carbon monoxide in blood samples from both bodies, which were sufficient to suggest ‘severe exposure’ to carbon monoxide.

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 said.

A 10-hour exposure would be ‘sufficient to cause carbon monoxide poisoning’, he added.

Professor Chilcott said that in the UK there are occupational limits about how much you are able to inhale, which would be ‘rapidly exceeded’ by spraying it in a hotel room

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.

The hearing was adjourned until Friday.