Four fire crews have spent the night tackling a fire in a building in Neston which was being used to store ten tonnes of tyres and plastic toys.
Wiltshire Fire and Rescue Service said today that arson was probably to blame.
Two crews from Corsham were called to the former factory at Brockleaze just after 7pm last night, after items inside the 30m by 15m building caught fire, causing thick plumes of black smoke.
They were joined by appliances from Chippenham and Melksham, supported by a water carrier from Royal Wootton Bassett, an incident command vehicle from Devizes and an operational support unit from Swindon.
More than 30 firefighters worked through the night on the blaze.
At 9am one pump was still on the site so that the situation could be monitored.
A fire service spokesman said the exact cause of the fire remains unknown but it is believed to have been started deliberately.