The crash happened near the famous Waddesdon Manor in Aylesbury, England.
A spokesman for Thames Valley Police, who were called to the incident shortly after midday, said: "Officers are now at the scene of an air accident near the village of Waddesdon near Aylesbury".
Four people have died in a mid-air collision between a helicopter and a two-seater aircraft, crash scene investigators have said.
An archived report by the Air Accidents Investigation Branch shows the pilot admitted the accident was caused by his "misjudgement and lack of experience".
The crash site is near, but not in the grounds of Waddesdon Manor.
Police chief Rebecca Mears said she could not yet give out the identities of the victims.
RAF Halton, 10 miles away, said no military aircraft had been involved.
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Both aircraft had taken off from nearby Wycombe Air Park, a spokesman said.
A spokesperson for the airstrip said, "Wycombe Air Park can confirm that at 12 pm (local time) today we were informed of an incident north-west of Aylesbury involving two aircraft from Wycombe Air Park".
Thames Valley Police, attending the scene, say "preservation of life" is a priority.
Buckinghamshire Fire and Rescue Service have seven vehicles responding to the incident, including fire engines and urban search and rescue vehicles.
"We sent a number of resources to the scene, including the Thames Valley air ambulance, two ambulance crews, two ambulance officers and a rapid response vehicle", said a spokeswoman.
SEVERAL people are feared dead after a plane and helicopter collided in mid-air near a Buckinghamshire manor house this afternoon.