More than 50 firefighters were called to the building, owned by garden centre suppliers Gardman, in Parsons Road late on Sunday (March 11 2018).
The roof appears to have collapsed and smoke could be seen for miles around on Monday afternoon.
Nobody is believed to have been injured but fire officers are expected to remain at the site for several days.
Deputy chief fire officer David Harding, from Northamptonshire Fire and Rescue Service, told BBC News: "It will be some time yet before investigators can gain access to the building however, police have launched an arson investigation."
He advised people living nearby to close doors and windows.
A number of surrounding roads at the Drayton Fields Industrial Estate have been closed by police.
Gardman issued this statement on their website, which said: “Shortly after midnight on 12th March a major fire broke out at the Garman warehouse in Daventry.
“At this time there are not believed to be any casualties and the Northants Fire Brigade are attending the site. The cause of the fire is unknown and is being investigated by the authorities.
“At this time we have no access to the site, a major incident team has been set up and we all updated you as and when we have more detail.”
Video footage of the fire was released by Northamptonshire Fire and Rescue Service.
*UPDATE* March 14 2018
Gardman has since updated its website with a new statement which reads:
"Yesterday, the emergency services spent a second day attending the scene. The fire is fully contained but the investigations are on-going. Emergency service presence has been scaled back over night and discussions about the demolition of the building will take place today.
Northamptonshire Police confirmed that a man arrested on suspicion of arson in connection with the fire has since been released on bail pending further enquiries. Whilst the fire has destroyed the Daventry warehouse, our business recovery plan is starting to take shape as we consolidate stock from other locations, receive stock from overseas and continue to source our core business of wild bird care locally. We would like to extend our thanks to all those who have offered their help and support whilst we pull this together."
GCA release statement regarding Gardman fire: March 16 2018
Chairman of The Garden Centre Association (GCA), Mike Lind, said: “The GCA are devastated for Gardman following the recent fire at their Daventry HQ.
“The fire has completely destroyed Gardman’s main UK warehouse and distribution centre.
“Gardman have worked extremely hard over the last few months to ensure they were able to provide the best products and services to all their customers in time for the busy spring season.
“Gardman are a long-standing and active member of the GCA who have always been fully engaged in the association’s activities as well as supporting a large number of GCA members’ businesses.
“The GCA extends its heart-felt thoughts and sympathies to all the company’s directors and staff and further offers its support and best wishes to the company in restoring its full business activity as soon as is practically possible.”