Crews from Bognor Regis, Littlehampton, Chichester, Worthing, Petworth and Southsea were called to the blaze before 5.40am.
Firefighters remain at the scene.
According to traffic reports, Downview Road is closed both ways between B2259 Felpham Way and Wroxham Way.
West Sussex Fire and Rescue Service has been approached for further information.
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