Two people have died and four others are seriously injured after a speedboat accident the ambulance service said on Sunday night.
A major search and rescue operation got under way after the incident in Padstow harbour, north Cornwall.
South Western Ambulance Service said two people had died and four were seriously hurt.
A spokeswoman for the Maritime and Coastguard Agency said: "Falmouth Coastguard was first called at 3.50pm when there were reports that six people had been thrown off a speedboat, and that the speedboat was out of control for a short time."
A Devon and Cornwall Police spokesman said officers were called to the scene in the Camel Estuary, near Padstow.
He said: "It's still an ongoing incident and we are working alongside the coastguard and ambulance service."
South Western Ambulance Service said they were called to reports that a speedboat had come into contact with a number of kayakers.
Two people were dead when the ambulance service arrived and another four people were airlifted to Derriford Hospital in Plymouth.
Ambulance service spokesman John Oliver said those injured were children and adults with leg injuries of various degrees of severity.
It is not yet known if they were in the speedboat or in the water at the time of the collision.
The coastguard spokeswoman said initial reports were that the boat had struck a number of people in the water and it was thought these might have been thrown off the boat.
The search and rescue operation was undertaken by RNLI lifeboats from Rock and Padstow, Cornwall, and helicopters from RNAS Culdrose, Cornwall and RMB Chivenor, Devon, plus three coastguard rescue teams from Polzeath, St Merryn and Newquay, all Cornwall.
Maritime and Coastguard Agency spokeswoman Jo Rawlings told Sky News later: "There are still search and rescue units on scene, but all those who were injured have been transferred to hospital."
She said it was not clear whether it was a commercial speedboat or privately owned - it is a popular area for speedboats, she said.
"It has been a busy day for the area around Padstow, it's been nice weather and a Bank Holiday weekend, a lot of visitors have descended on the coast."
The hospital referred callers to the police, who had no more information yet.