Officers from Havant Roads police unit from Hampshire Constabulary are appealing for witnesses to an incident on the Chawton roundabout in Alton on Sunday 30 June.
Between 12.35pm and 12.45pm a group of five motorbikes were driving round the Chawton roundabout, A31.
During this incident, a black Mazda entered the roundabout from Chawton Village. No collision took place, but one of the motorbikes skidded on the grass verge before coming to rest on the A32 approach to the roundabout.
The 37-year-old man who was riding the motorbike sustained serious injuries to his leg.
Officers who are investigating are appealing for anyone who witnessed this, and saw how the motorcyclist came to leave the road.
PC Chris Dawes said: “We know that another vehicle stopped at the scene at the time but had left before police arrived.
“We believe this person would have witnessed what happened, so if this was you please give us a call”.
Anybody with information about this incident is asked to call 101, quoting 44190225794, or contact Crimestoppers anonymously on 0800 555 111.