An aircraft and a helicopter have crashed in mid-air over Buckinghamshire, with a number of casualties reported.
Emergency services were called to the site near Waddesdon Manor, near Aylesbury, at 12:06 GMT.
The Air Accidents Investigation Branch (AAIB) has sent a team to investigate.
A Wycombe Air Park spokesman said both aircraft came from the airfield near High Wycombe.
LIVE: Updates on ‘mid-air’ crash in Buckinghamshire
The AAIB said the plane involved was a Cessna.
South Central Ambulance Service said it received a call for a “mid-air collision” between a helicopter and an aircraft in Upper Winchendon, near Aylesbury.
It said: “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”.
Mitch Missen, an off-duty firefighter, witnessed the crash from his garden.
He said: “I looked up and saw as both collided in mid-air, followed by a large bang and falling debris.
“I rushed in to get my car keys and en route called the emergency services, who I continued to give updates as to its whereabouts.
“Unfortunately, I wasn’t able to locate the actual crash site but directed police, fire and ambulance as best I could. Once they were on the scene, I returned home.”
There were a number of road closures following the crash but they have since been lifted.
Seven fire vehicles from Aylesbury, Haddenham, Oxfordshire and Berkshire went sent to the scene.
A spokesman for Bucks Fire and Rescue Service said 30 members of staff in fire engines and urban search and rescue vehicles attended.
He added: “I understand it is in a wooded area near the manor.”
The Thames Valley air ambulance, two ambulance crews, two ambulance officers and a rapid response vehicle were also sent to the scene.
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 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.”
BBC News Online reporter Phil Shepka at the scene
When I got to the scene it was frantic, as media across all outlets, local and national, assembled.
You cannot see the crash site as the woodland is so dense but now the light is dimming, you can see light coming from where the AAIB and police are.
The police presence has been very visible throughout the day and they have now reopened roads which were closed.
There is still no word from the force over the number of people involved in the incident.
A spokesperson for the National Trust-owned Waddesdon Manor said the crash had not happened in its grounds, but 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.
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