Wreckage from both aircrafts reportedly landed near the grounds of the Rothschild family Waddesdon Manor house in Buckinghamshire.
South Central Ambulance Service said the alarm was raised at 12.09pm (1209 GMT) local time "to a mid-air collision involving a helicopter and an aircraft".
Air crash investigators have been called to the collision site, in a dense woodland, alongside paramedics, firefighters and Thames Valley Police. Officials confirmed there had been casualties, but were unable to give any further details. Staff at the manor are understood to have assisted at the scene of the crash.
Buckinghamshire Fire and Rescue Service said they now have seven vehicles responding to the incident, including fire engines as well as urban search and rescue vehicles.
Fire services also on social media: "Firefighters are now assisting other emergency services at scene of air accident near Waddesdon".
The Air Accidents Investigation Branch said Friday it was sending a team to Aylesbury, Buckinghamshire to investigate.
Bucks Fire and Rescue is also in attendance, having sent seven vehicles including urban search and rescue.
"Our priorities today remain with investigating the next of kin, finding out who they are, informing them and supporting them with specialist officers as we progress the investigation here on site", she said in a statement.
"Disruption to road network around Waddesdon likely for the rest of the day".
Road disruptions around Waddeson are expected for the rest of the day.
RAF Halton, which is around 10 miles away, said no military aircraft had been involved.
Roger Nock, a pilot, tweeted: "Was flying near Aylesbury earlier".
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