Road workers discovered an “unusual but sophisticated” cannabis farm growing in a drainage tunnel under the M1 motorway.

More than 240 plants were found in the pipe that runs the width of the motorway near the village of Watford in Northamptonshire.

Police described the set-up as “one of the most unusual locations” they had ever discovered cannabis being grown in but said it “demonstrates the length criminals will go to produce this high-value commodity”.

The drainage culvert – a closed tunnel usually used to move water from one side of the road to the other – had been boarded up to try to hide the unorthodox farm.

Maintenance workers from National Highways discovered the plants and reported the find to police.

Insp Jonathan Davis-Lyons, of Northamptonshire Police, said: “Once inside the drainage culvert, we found a sophisticated set-up which included extensive shelving to ensure the plants were kept above water and power diverted for lighting and other equipment needed to maintain the plants.”

Heavy rain and the fact that the nearest road access to the tunnel was around a mile away made the cannabis factory difficult to dismantle but a local landowner provided police with transport and trailers to help.

Officers said no one had been arrested and they were keen to hear from anyone with information about who had been growing the plants.