About eight fire engines have been called to Milton Malsor to deal with a large fire at four business units.
Firefighters said they saw a “huge” plume of smoke and heared “popping” sounds at the scene, which was at JBJ Business Park in Northampton Road.
Eight fire engines and two aerial units tackled the blaze after being called out at 10.45am.
The fire broke out in a Protiler Tools storage unit and spread to neighbouring units including Carpet Line, Taylor Carpets and PLC Transport.
Eye witnesses said the smoke could be seen from Long Buckby and Rushden.
Brigade manager Dawn Whittaker said: “I arrived at the scene early doors and began tackling the fire in the units. We believed the fire was going out as the smoke had started to turn white but I was forced to pull firefighters back as a big black cloud of smoke emerged from the roof.
“The fire had spread to a narrow concealed unit at the back of the other units that we didn’t know was there. It contained paint thinners and flammable material and the whole thing went ‘pop’.”
No roads were closed because of the fire and nobody was injured, although an elderly couple in a nearby house were evacuated.
Some units had staff working at the time of the fire but all escaped unharmed.
Four of the eight units on the site have been gutted by the fire.
The son of one of the business owners said on Facebook he believed the blaze was started accidentally and may have been caused by an extension lead. However, the fire service has not started any official investigations.
Incident commander Rob Porter said: “We are now dampening down and expect to be on the scene for until between 5pm and 7pm.
“We have had units here from Daventry, Leicester and Buckingham today, but won’t know the cause of the fire until the investigators have taken a look.”