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.
South Central Ambulance Service 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.
“There have been a number of casualties at the scene, but at this stage this is all we are able to confirm.”
Thames Valley Police said the priority was “saving lives”. The force said there was expected to be disruption in the area for the rest of the day.
Seven fire vehicles from Aylesbury, Haddenham, Oxfordshire and Berkshire are at the scene.
A spokesman for Bucks Fire and Rescue Service said they had sent fire engines as well as urban search and rescue vehicles.
He 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.”
Hayley O’Keefe, from The Bucks Herald, said on Twitter a “plume of smoke” could be seen close to Waddesdon Hill after the crash.
The Rev Mary Cruddas from St Mary Magdalene church, Upper Winchendon, said she had been to the crash site to see if she could be of any help.
She said: “The area where it happened is off road and difficult to get to.”
A spokesperson for the National Trust-owned Waddesdon Manor said the crash had not happened in its grounds, but said staff helped direct the emergency services to the scene.
Wycombe Air Park, also known as Booker Airfield, is about 20 miles (32km) away from the site of the crash and offers flight training.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: