A massive plantation of 40,000 marijuana plants has been discovered by police in a cornfield in eastern Ontario.

The plants were found Sept. 18 at a farm on B Line Road in Laurentian Valley Township near Pembroke, Ont., about 150 kilometres west of Ottawa, during the execution of a search warrant, said an Ontario Provincial Police news release that described the bust as "the largest in Canada's history."

It is one-third larger than the infamous 2003 operation at the former Molson brewery in Barrie, where police seized 30,000 plants.

Sgt. Kristine Rae said it took officers all weekend to seize the plants, which had a street value of approximately $40 million and were hidden between rows of corn.

"That is huge, trying to remove all that by hand," she said.

Some of the marijuana found last week was in the process of being harvested, and a double garage behind the farmhouse had been set up to dry the plants, the release said.

The plants were being watered via a system of plastic pipes running from a pond and an above-ground swimming pool.

There have been no arrests in the case, but Rae said the investigation is continuing.

"There's persons of interest but it will take some time to follow through," she added.