Two fire engines from Braintree were called to tackle a blaze on the back of a transporter lorry in the middle of the night.
The lorry, which was carrying two vehicles, was parked beside Great Leighs racecourse, on the A131, to let the driver rest when the fire broke out.
Fire engines were called at 3.44am and firemen wore breathing apparatus and used one hose reel to tackle the flames.
The fire was extinguished by 4.22am.
Firecrews called the police because of the suspicious nature of the fire.
An investigation into the cause of the fire will be carried out.