Emergency services were called at 3.38pm on Monday, March 23, to a collision between a car and a girl, believed to be around 4-years-old.
Guildford Road and Bishopric have been closed while emergency services deal with the situation. It is believed the car involved has left the scene.
Sussex Air Ambulance has also been called to the scene. It is also reported Blackbridge Lane is closed.
Anyone who saw what happened is urged to contact 101 quoting serial 942 of 23/03.
More as the story develops.