South Wales Fire and Rescue Service received a call at 3.18am to put out the fire at the potato crisp factory operated by Sirhowy Valley Foods, Pen Y Fan Industrial Estate, Croespenmaen, Crumlin.
When they arrived they discovered what a fire service spokesman called a “well-developed fire”, which was already engulfing three-quarters of the building.
"On arrival, our crews encountered a very well developed fire, involving about 75% of the building,” a spokesman told FoodManufacture.co.uk. “Efforts were focusing on preventing the blaze from spreading to other units on the industrial estate.”
The fire was said to have spread quickly because the building contained fats and oils used in the manufacturing process.
Night workers at the factory were evacuated as soon as the blaze broke out.
Nearby residents were advised to keep their doors and windows shut.
Fire-fighters remained at the scene all day damping down the fire and ensuring the building was safe, added the spokesman.
An investigation into the cause of the blaze has been launched.
Environmental health officers were said to be monitoring the situation while an ambulance service hazard response team had been placed on standby.
Sirhowy Valley Foods is owned by Northern Ireland snack company, Tayto Group.
Paul Allen, CEO of Tayto Group, said: “We are deeply relieved that all our emergency procedures worked well and that all our nightshift staff are safe. Our focus now is on what we can do for those staff, who are naturally concerned for their jobs."
Allen added that the group would be working hard to ensure minimal disruption to customers and that they would be considering all options available, including their other factories.