An aircraft and a helicopter have crashed in mid-air over Buckinghamshire, the Air Accidents Investigation Branch (AAIB) said.
Emergency services were called to the site near Waddesdon Manor, near Aylesbury, at 12:06 GMT. The AAIB said it was sending a team to investigate.
Ambulance crews said there had been a number of casualties.
A Wycombe Air Park spokesman said both aircraft involved came from the airfield near High Wycombe.
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