Three lanes were shut on the A1M following a collision.
There were delays of around 20 minutes northbound between Sawtry and Stilton, with only one lane open.
Lots of water was on the road, according to police.
A spokeswoman said: "At approximately 9am today a police car was involved in a one vehicle collision on the A1M northbound outside Sawtry. Three lanes are currently closed, with one operating. Injuries are not believed to be serious. We will tweet when the lanes are reopened."
All of the emergency services were at the scene.
11am - the road is now fully open