An aircraft and a helicopter are believed to have crashed in mid-air over Buckinghamshire.
The Air Accidents Investigation Branch said it was “sending a team to investigate a mid-air collision involving an aircraft and a helicopter near Aylesbury, Buckinghamshire”.
Emergency services were called to the site, near Waddesdon Hall, at 12:06 GMT.
Police said the priority was being placed on “saving lives”.
Thames Valley Police said there was expected to be disruption in the area for the rest of the day.
Police, the ambulance service and fire crews from Aylesbury, Haddenham, Oxfordshire and Berkshire are at the scene.