Lincolnshire Police are appealing for witnesses to a crash involving a private ambulance and three other cars on the A17 at Byards Leap near Sleaford.
The incident happened at around 2.20pm on Saturday, March 10 between a white Renault private ambulance, a green Land Rover Discovery, a grey Isuzu, and a green Nissan Micra.
A woman in her 40s was seriously injured and was taken to Queen’s Medical Centre in Nottingham, where she remains for treatment.
The road was closed for some 12 hours as police investigated the crash and then cleared the road.
Anyone who was in the area around the time of the collision, or saw it happening, is asked to call police on 101 quoting incident 218 or March 10.