At least one person is understood to have died in a motorway pile-up involving 30 cars.
Fire crews, police and air and road ambulances rushed to the M40 between junctions 9 and 10, near Bicester in Oxfordshire, just before 7.50am after dozens of vehicles were involved in a crash.
A South Central Ambulance Service spokeswoman said they are currently assessing more than 30 patients.
The motorway has been closed northbound between the junctions, while a couple of southbound lanes are shut along the same stretch.
An ambulance spokeswoman said it is too early to know the nature and severity of the injuries suffered by drivers.
She said: “It is quite a major collision. We were called at 7.48am to reports of a multiple road traffic collision on the M40, junctions 9 to 10 northbound.
“I understand that there are about 30 vehicles involved. We are currently assessing in excess of 30 patients.
“We have a lot of resources on the scene - ambulances, our helicopter ambulance, rapid response vehicles, our doctor and our hazardous area response team.
“Fire and police are also at the scene.”
Thames Valley Roads Policing said on Twitter: “M40 northbound J9 will be closed for a number of hours following a fatal collision. Officers are working to relieve the traffic stuck northbound.”
A Highways Agency spokesman said all lanes north along the stretch are closed and may remain shut for several hours as police investigate the crash.
A couple of lanes south are also closed.