Two guests evacuated from the Egyptian hotel where a British couple died last week claim they have an infection known as shigella.
The mother and daughter were among hundreds who left the resort in Hurghada after John and Susan Cooper died suddenly just hours apart.
The pair who have fallen ill are part of a family of four, all of whom became unwell on the Thomas Cook holiday, their lawyer Nick Harris said.
Since the deaths of Mr Cooper, 69, and his wife, 63, from Burnley, Lancashire, dozens of guests have claimed they were taken ill at the Steigenberger Aqua Magic hotel.
The mother and daughter were among hundreds who left the resort in Hurghada after John and Susan Cooper (pictured) died suddenly just hours apart
Shigella is a contagious cause of food poisoning which is said to kill hundreds of thousands a year worldwide.
Samples showed the mother and daughter had the bacterial infection, a health officer told The Daily Telegraph.
This development follows dismissals from the hotel manager of any increased incidences of illness among guests.
Mr Harris, from law firm Simpson Millar, told the newspaper: ‘This is the first indication of a pathogen and completely changes the game.
‘Shigella is a communicable disease. In my experience it is often food borne but it’s also water borne so you can get it through contaminated water.’
Mr and Mrs Cooper’s daughter, Kelly Ormerod, has questioned how they could have fallen ill so quickly, saying there is ‘something suspicious about their deaths
Thomas Cook has commissioned its own tests into air conditioning and food hygiene at Aqua Magic, thought the organisation has not been granted access to the Coopers’ room. Pictured: The Steigenberger Aqua Magic Hotel
Other guests have reported falling ill with vomiting and upset stomachs during their stay at the hotel this month.
Thomas Cook has commissioned its own tests into air conditioning and food hygiene at Aqua Magic, thought the organisation has not been granted access to the Coopers’ room.
A spokesman from the firm insisted that its customers’ safety and wellbeing was its first priority, adding that Steigenberger Aqua Magic was last audited in July.
Mr Harris said that there were ‘a number of competing interests’ in identifying what was behind the Coopers’ deaths.
Thomas Cook admitted a high level of illness had been reported at the five-star hotel but would want to protect its reputation, he said, while its German owners initially claimed no one had been taken ill.
Local authorities would be very wary of what a sickness scandal could do to the tourism trade, he added.
The cause of death of Mr Cooper, 69, and his wife, 63, has not yet been determined.
Their daughter, Kelly Ormerod, said her parents had been ‘perfect health’ before going to bed before they were found dying in their hotel room the following morning.
Egyptian authorities initially claimed Mr Cooper had died from a heart attack and his wife then collapsed from grief.
Their bodies will be repatriated next week, Egypt’s minister of tourism confirmed.
Rania Al-Mashat explained that forensic pathologists are carrying out ‘detailed autopsies’ which would conclude next week.
There were no toxic gas emissions or leaks in the room, according to a statement from prosecutor Nabil Sadeq.
Thomas Cook’s chief executive, Peter Frankhauser, has flown to Egypt to meet prime minister Dr Mostafa Madbouly about the deaths.