Carbon monoxide poisoning is thought to have killed a mother and her 10-year-old daughter on a boat in Cumbria, the BBC has reported.
A man, 39, who was also on the privately owned vessel on Windermere was taken seriously ill after the incident on Monday afternoon.
Kelly Webster, 36, and Lauren Thornton, from the Leyland area of Lancashire, were airlifted to Royal Lancaster Infirmary where they both died.
Post-mortem examinations are due to be carried out later.
It is understood one theory involves the use of a generator on the boat, which may have been faulty.
Cumbria Police said officers were called to reports that three people on the boat were having serious breathing difficulties.
The incident, at a jetty near Bowness, was witnessed by Dragons’ Den star Duncan Bannatyne, who remarked on the police and ambulance activity to his followers on Twitter.
He later posted a photograph of the scene, saying: “Tragic accident over there I am afraid.”
Other eyewitnesses reported seeing firefighters entering the boat wearing breathing masks.
Nigel Wilkinson, from Windermere Lake Cruises, said public cruisers were diverted while the emergency services worked at the scene.
He said police, fire, ambulance, lake wardens, community first responders and the Great North Air Ambulance and the North West Air Ambulance were all involved.