The A605 has been closed at the junction of the B1095 near Stanground after a crash.
A Cambridgeshire police spokesman said the collision was thought to have been caused when one driver suffered a ‘medical episode’ at the wheel at about 2.20pm.
An East of England Ambulance Service spokesman said: “An ambulance crew, ambulance officer and East Anglian Air Ambulance attended.
“They treated a man, believed to be in his 70s, who had gone into cardiac arrest.
“He was taken to Peterborough City Hospital in a critical condition.”