An 11-year-old boy was airlifted to hospital following a crash on the A47 on Saturday night, in which four other people were also hurt.
An investigation has been launched into the incident, which happened at North Runcton, close to the New Road junction, at around 8.20pm.
Police say a red MG ZR, which was travelling towards Lynn, was involved in a collision with a Nissan Qashqai heading in the opposite direction.
Three fire crews, all from Lynn’s stations, were called to the scene to help free casualties from the wreckage.
The boy, who had been travelling in the MG, was airlifted to Addenbrookes Hospital in Cambridge with serious injuries. Police say he is still being treated there.
Two other people in the car were taken to Lynn’s Queen Elizabeth Hospital, one with serious injuries. Two people who were travelling in the Nissan suffered minor injuries.
The road was closed for more than five hours while investigations took place at the scene.
Police say they want to hear from anyone who saw the collision, or either of the vehicles involved prior to it.
Anyone with information is asked to contact Sgt Peter Howlett or PC Bob Hardingham, of the county’s roads policing unit, on 101.