An aircraft and a helicopter have crashed in mid-air over Buckinghamshire, the Air Accidents Investigation Branch said.
The AAIB 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 Manor, 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.
A spokesman for Bucks Fire and Rescue Service said: ‘There are 30 fire service staff there from all counties, working alongside police, ambulance and the AAIB.
‘I understand it is in a wooded area near the manor.’
Waddesdon Manor said the incident had not happened in its grounds.