Gas workers visited an old Rushden factory to disconnect it - only to find more than 1,500 cannabis plants.
The drug farm in Park Road had been undetected and was only discovered because utilities bills had not been paid.
British Gas staff arrived at the disused factory site, set back from the road near Rushden's town centre, at about midday on Thursday (June 20) to disconnect the supply.
But they were met with a large number of cannabis plants, lights and watering equipment and called police.
Officers arrived and spent hours bagging up the 1,509 plants so they can be destroyed.
A police spokesman said: "There were none of the usual signs such as [the] smell and it would appear to have been in the early stages of production due to the growth of the plants.
"Enquiries are now ongoing to trace the owners/tenants/users of the factory which British Gas are helping us with."
Had the cannabis factory remained undiscovered the plants had the potential to grow class B drugs with a street value of about £1m.
The cannabis factory is the latest to be found in north Northamptonshire.
Earlier this month a £190,000 drug farm was found in an abandoned curry house in Kettering.
And days later a huge factory - with the potential to grow drugs worth almost £3m - was discovered right under the noses of Kettering shoppers in the old bingo hall.