Four people have been killed after a mid-air collision between a helicopter and an aircraft.
Both of the aircraft are understood to have come from Wycombe Air Park and collided near Waddesdon, in Buckinghamshire, shortly after midday.
Investigators have refused to release details of the gender or identity of the victims, saying their first priority is finding and informing the next-of-kin.
The plane was a Cessna 152 with capacity for a pilot and one passenger. Built in 1982, the plane sustained substantial damage during a previous crash in 1993 in a Cornish airfield.
Air crash investigators have been called to the crash site, in a dense woodland, alongside paramedics, firefighters and Thames Valley Police, who said the first priority was "preservation of life".
A spokeswoman for South Central Ambulance Service (SCAS) said: "There have been a number of casualties at the scene, but at this stage this is all we are able to confirm."
A spokesman for the airfield, near High Wycombe, said: "Wycombe Air Park can confirm that at 12 o'clock local today we were informed of an incident north-west of Aylesbury involving two aircraft from Wycombe Air Park."
Police are co-ordinating the response to the crash and warned there will be disruption to surrounding roads for the rest of the day.
Crash site investigators are expected to be deployed at the scene until at least Monday to try and determine the cause of the accident.
The collision took place close to the village of Waddesdon, about 20 miles from the Wycombe Air Park, at 12.06pm.
The SCAS spokeswoman said: "We received the call at 12.09pm, for a mid-air collision involving a helicopter and an aircraft in Upper Winchendon, near Aylesbury.
"We sent a number of resources to the scene, including a Thames Valley air ambulance, two ambulance crews, two ambulance officers and a rapid response vehicle."
Waddesdon Manor, on the Rothschild family-owned estate near the crash site, described the crash as a "tragedy" and added it "did not happen at the manor nor have there been any casualties at Waddesdon Manor".
Waddesdon Manor is managed by the Rothschild Foundation, a family charitable trust, on behalf of the National Trust, who took over ownership in 1957.
Wycombe Air Park, also known as the Booker Airfield, offers training to rookie pilots.
RAF Halton, which is around 10 miles away, said no military aircraft had been involved.
A spokesman said: "We can confirm that neither of the aircraft concerned has a connection with either our air force nor the military, and this is as much as we know at this time."
Wycombe Air Park is also known as Booker Airfield and sits around 20 miles away from the site of the crash.
In May, a man was left fighting for his life after the helicopter he was travelling in with two other men crashed at the site.