Black smoke from the inferno was visible for miles around after County Plastics' open storage yard at Barnfield Mill on Union Street was torched.
Fire fighters estimate the flames destroyed around 80 to 90 per cent of the business' stock.
They were called to the blaze shortly before 6pm on Saturday evening and remained on the scene until around 4.30am.
Crews from Atherton and Leigh station put out the flames with the help of a hydraulic platform and there was significant disruption to traffic while the blaze was brought under control.
Watch commander Terry Peers, of Atherton station, said: "There was thick black smoke billowing into the sky and it was clearly visible from Asda in Leigh.
" When we got there it was quite an inferno it had drawn a crowd of spectators because the flames were spectacular.
"It started in the plastic stored on pallets next to the fence and it would be easy for someone to reach through and start the fire.
"Plastic is quite toxic when it burns but fortunately it was mainly rising into the air rather than blowing sideways.
"We tried to get close to it but we couldn't because it was so hot.
"We had to get the hydraulic gear in to get over the top of it and it took copious amounts of water to put it out.
"Eventually we got hold of an owner and he managed to clear some nearby plastic out of the way with his fork-lift truck to create a fire-break."
The fire crews have recorded the incident as a case of arson and passed their information to police.
The owner of the business declined to comment.
One neighbour, whose property overlooks the yard, said: "We were out all night but when we came back at about 4am we could still see all the smoke and smell the burning."