More than 100 firefighters were tackling a blaze on an industrial estate, Surrey Fire Brigade said.
The brigade was called shortly before 1900 BST to the fire in the Thorpe Industrial Estate near the M25 motorway at Egham, Surrey.
A total of 19 fire engines and up to 120 firefighters were there but there were no reports of any casualties.
An eyewitness said the fire had swept through the MY Healthcare factory, which makes boxes for medicines.
Russell Smith, a welder at a nearby firm, said: "I saw the smoke and went out to have a look.
"I saw that there was smoke billowing from all the chimneys and windows at the factory opposite.
"There were flames coming up through the roof and the people who work there had got out."
Mr Smith, 48, said firefighters were using water from a nearby lake on the industrial estate to help douse the flames.
He said workers at the factory had told him they had spotted the fire in the stores, which is packed with card, and it had quickly taken hold.
A fire brigade spokesman said the fire had been isolated and would not spread to any other buildings.
Police closed Crabtree Road and Ten Acre Lane and both roads are expected to remain shut for some time.
Officers are expected to be on the scene all night and for much of Sunday.
The factory, which makes packaging and boxes for pills, has two floors and is about the size of a football pitch.
The fire brigade spokesman said the owners of the building had been informed. He added: "It's at quite an early stage. We have got the fire surrounded now so we are confident it will not spread to other buildings."
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