Urban planners have dreamt of a subway line running south along Brooklyn’s Utica Avenue since at least 1910.

Now Mayor Bill de Blasio is calling on the Metropolitan Transportation Authority to study the idea in his wide-ranging “sustainability blueprint” that was released on Wednesday.

The city says the line would run through one of the most densely populated areas that currently has no clear access to the subway system. 

De Blasio, however, did not propose any way to pay for it.

Funding, cost and competing projects, in part, were problems that plagued plans for a Utica Avenue line in the early 20th century, according to a 2013 book, The Routes Not Taken. A real estate owner, William E. Harmon, who stood to profit from transportation near his Brooklyn properties, was a prominent advocate for the rail in 1910. But as the decades went on, issues arose, such as new community opposition to an elevated train that would be cheaper to build than a subway, and a desire to build a Nostrand Avenue line.

The Utica Avenue plans were revived in the 1960s as part of the MTA’s “New Routes” lines, according to the book, but the project and others “faded away” during the fiscal crisis of the 1970s.

De Blasio’s OneNYC plan also proposes improvements that would make it possible to operate trains more frequently along the subway lines as well as upgrades to subway stations to relieve crowding.

The MTA didn’t directly respond to a request for comment on the proposed study of the Utica line.

“The MTA and the city of New York are aligned on the importance of mass transit to keep the city thriving,” the MTA said in a statement. “We look forward to working with the city as we seek full funding for our 2015-19 Capital Program.”