An aircraft and a helicopter have crashed in mid-air near Aylesbury, Buckinghamshire.
Thames Valley Police, attending the scene, say “preservation of life” is a priority.
South Central Ambulance confirmed a “number of casualties” at the scene.
The crash is understood to have happened in Upper Winchendon, close to Waddesdon Manor, though a spokeswoman for the estate said it had not happened “on its grounds”.
Both aircraft came from Wycombe Air Park, also known as Booker Airfield, near High Wycombe, which offers flight training.
A spokesman for the airstrip, near High Wycombe, said: “Wycombe Air Park can confirm that at 12pm local today we were informed of an incident north-west of Aylesbury involving two aircraft from Wycombe Air Park.”
A Thames Valley Police statement confirmed: “Thames Valley Police is currently at the scene of an air accident near the village of Waddesdon near Aylesbury.
“The force is coordinating the response to the incident which was reported at 12.06pm today (17 November).
“The Air Accidents Investigation Branch has been informed and staff are en route to the scene. Fire and ambulance services are also in attendance and preservation of life is first priority.
“In consequence of this incident it is expected that there will be some disruption to the road network around Waddesdon for the rest of the day.”
RAF Halton, 10 miles away, said no military aircraft had been involved.
Bucks Fire and Rescue is also in attendance, having sent seven vehicles including urban search and rescue.
A statement on Twitter read: “Firefighters currently assisting other emergency services at scene of air accident near Waddesdon. Air Accidents Investigation Branch informed.
“Disruption to road network around Waddesdon likely for the rest of the day.”
The AAIB has sent a team to investigate the crash.
Thames Valley Air Ambulance are also at the site of the crash, a spokesman said.
A spokesman for Waddesdon Manor told Sky News the crash had happened near Upper Winchendon and staff from the estate had gone to the scene in order to help direct traffic.
No one on the estate was harmed and the Manor will be open as usual tomorrow.
Roger Nock, a pilot, tweeted: “Was flying near Aylesbury earlier. Skies around here are so busy.”