According to West Sussex Fire and Rescue, firefighters from Bognor and Chichester were called out at 3.40pm following reports of a double decker bus ‘well alight’ on the westbound side of the carriageway.
Fire crews attended the scene with two breathing apparatus where there was ‘smoke issuing from the bus’, fire services said.
It has been confirmed that the fire was in the engine compartment of the bus and there has been no damage to the bus itself.
The bus was empty and no injuries have been reported, fire services have said.
Travel reports show that the incident is still blocking one lane with heavy traffic between the A285 junction and Portfield roundabout.
West Sussex Fire and Rescue has said drivers should avoid if possible.