A woman and a young girl have died and a man is seriously ill after suffering breathing difficulties on a boat.
The alarm was raised at around 4pm on Sunday when Cumbria Police and an ambulance were called to a private boat at Lake Windermere in the Lake District.
A 36-year-old woman and a 10-year-old girl, both from the Leyland area of Lancashire, were treated at the scene and then air-lifted to Royal Lancaster Infirmary, where both have since died.
The man, who was also on the boat, is still receiving treatment at the Royal Lancaster Infirmary, Cumbria Police said.
A spokeswoman for Cumbria Police said: "Cumbria Police can confirm that at approximately 4pm today, they attended an incident at Lake Windermere, where it was reported that three people on a private boat on Lake Windermere were having serious breathing difficulties.
"Police attended along with an ambulance crew.
"A 36-year-old woman and a 10-year-old girl, both from the Leyland area, were treated at the scene and then air-lifted to Royal Lancaster Infirmary. Unfortunately both have since tragically died. A man, who was also on the boat, is still receiving treatment at the Royal Lancaster Infirmary."
The spokeswoman continued: "Police are currently investigating the circumstances of this tragic incident and trying to establish the full facts about what has happened. The coroner has been informed."
Cumbria Police said they were unable to confirm reports that the deaths were the result of carbon monoxide.
The unfolding incident, at a jetty near Bowness, was witnessed by businessman and television personality Duncan Bannatyne who remarked on the police and ambulance activity to his followers on Twitter. The Dragon's Den star later posted a photograph of the scene, saying: "Tragic accident over there I am afraid."