City of London Police discovered 826 plants in the basement of a commercial building following reports of a strong smell of the class B drug.
The cannabis factory, in Throgmorton Street, is the first ever found in the City of London.
It has since been dismantled and two people have been arrested.
Police said the initial discovery was made on the afternoon of 13 January and a warrant was obtained to enter the premises the following day.
Temporary Detective Inspector Andy Spooner, who is leading the investigation, suggested the drugs operation had been set up because the area has been quiet during the coronavirus pandemic.
He said in a statement: “This is the first cannabis factory in the City, no doubt being set up in response to fewer people being out and about during the pandemic who might have noticed any unusual activity.
“However, this demonstrates that City of London Police continues to actively police the Square Mile, bearing down on any crime committed here.
“We have made two arrests and our enquiries continue.”
Additional reporting by PA
Watch: Thai hospital serves up cannabis-infused dishes