The air ambulance has been called after a car careered into a ditch beside the A14 near Huntingdon.
The incident occurred at about 11am and saw a car spin off the road and into a ditch, with reports of four people suffering injuries.
Highways England says the A14 is blocked westbound between junctions 20, at Ellington, and 21, at Brampton Hut, and added that congestion is beginning to build in the area.
One lane is reported to have been closed while emergency crews deal with the incident and recovery vehicles are called.
Motorists are being advised to avoid the area, with delays expected until at least 1.30pm.