A building with links to animal experiments was engulfed in flames last night.
Firefighters worked though the night to control the blaze at the warehouse in Henfield Road, Small Dole, and were still there this morning.
A section of the road was closed off while fire crews doused the flames, causing rush-hour congestion.
The building is understood to be a vehicle store for the controversial Huntingdon Life Science group which tests on animals.
Firefighters cordoned off the building and closed Henfield Road, Small Dole, soon after they arrived at 11.30pm yesterday.
They discovered a number of cylinders and vehicles on fire.
Crews from Burgess Hill, Shoreham, Steyning, Keymer, Henfield and Horsham used powerful hoses to control the blaze from a safe distance.
This morning two crews were still cooling down the cylinders, believed to contain acetylene, to make the building safe.
Rush hour traffic was being diverted away from the badly damaged 15 metre-long corrugated steel building.
A fire brigade spokesman said: "As yet, we do not know what caused the fire."
Fire officers were due to start an investigation later today.
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