Police found 50 of the chest-high plants being cultivated under heat lamps and an irrigation system in a bungalow at the park in Windsor.
But theme park bosses say the vacant building is inaccessible to the public and “appears to have been accessed via the Crown Estate” - land belonging to the Queen.
Astonished workers discovered the professional set up while looking for asbestos in what is understood to be derelict staff accommodation yesterday.
A spokeswoman for Legoland said: “Following routine checks, we can confirm that illegal substances were found in a derelict cottage outside of the LEGOLAND Park boundary.
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“The building in question is inaccessible to the public and appears to have been accessed via the Crown Estate.
“We have notified Thames Valley Police and we will continue to support their ongoing investigation.”
The cottage containing the plants is on the 215 acres owned by Legoland, but outside the theme park boundary.
It is situated near the entrance to the park and kennels where visitors can leave their dogs.
A spokesman for the police said: “Thames Valley Police officers were called at about 11.15am today following reports that cannabis had been found at a property in the Legoland grounds in Windsor.
“Officers are currently at the site where cannabis plants and equipment used in the production of cannabis has been located.
“No arrests have been made and an investigation is being carried out.”