A 51-year-old man and his eight-year-old daughter were killed and four other members of their family airlifted to hospital after a speedboat accident, police have confirmed.
Devon and Cornwall Police said the family from London was on holiday in Cornwall over the Bank Holiday weekend when the accident happened in the Camel Estuary near Padstow.
A 39-year-old woman, a four-year-old boy and two girls aged 10 and 12 were airlifted to Derriford Hospital in Plymouth, where they are being treated for "a range of injuries, some of which are life changing", police said.
The Maritime and Coastguard Agency (MCA) said in a statement on Sunday: "Falmouth Coastguard was first contacted at 15.48 this afternoon reporting that six people had been thrown from a speedboat approximately half a mile off Padstow."
MCA spokeswoman Jo Rawlings told Sky News: "It is believed the boat was out of control."
The boat, a rigid inflatable vessel, was owned by the family, police said.
South Western Ambulance Service earlier said it was thought the speedboat hit some kayakers near the popular holiday resort, however this was subsequently ruled out by the MCA.
The MCA said: "We do not believe at this time any other vessels, kayaks or people were involved in this incident, other than those in the speedboat."
The man and girl were dead when the ambulance service arrived.
Ambulance service spokesman John Oliver said the injured suffered leg injuries of various degrees of severity.
RNLI lifeboats from both Padstow and nearby Rock were launched as part of the rescue operation, and search and rescue helicopters from two naval bases were also ordered to attend the scene.
Witnesses claimed heroic actions by passers-by prevented more lives being lost.
Writing on Facebook, Maria Chase said: "We were down in Padstow today and saw the tragedy and saw a hero who saved more lives by jumping on to an out-of-control speedboat from another boat."
Charlotte Jacobs took to Twitter to write to the RNLI, saying: "We witnessed incredible bravery in Padstow today during the tragic accident that unfolded. The men involved deserve medals."
Celebrity chef Rick Stein, who has four restaurants in Padstow, tweeted: "Our thoughts are with injured and bereaved involved in the tragic accident in Padstow this afternoon."
Fellow chef Paul Ainsworth wrote on Twitter: "Very sad day in Padstow, our hearts go out to all family and friends involved. Life can be so cruel."