Mayor Bill de Blasio wants to offer prekindergarten to every 3-year-old child in New York City, he said Monday in one of his most ambitious education announcements to date.

Calling the initiative “3-K for All,” de Blasio said the plan will start by expanding pre-K seats for younger children in District 7 in the Bronx and District 23 in Brooklyn over the next two years and encouraging more families to enroll in existing seats. The plan builds on de Blasio’s signature education initiative — a push to provide free pre-K to every 4-year-old in New York City — which he highlights as a major success.

“We have proven through the growth of Pre-k for All that it can be done, and it can be done quickly,” de Blasio said at a press conference Monday at P.S. 1 in the Bronx.

The latest initiative will take a while to reach every 3-year-old in the city. The city plans to fund eight districts on its own by 2021, but also wants to raise enough outside funding to make it universal by that time. Once fully rolled out, the city expects to serve 62,000 children in 3-K at a cost of more than $1 billion — though de Blasio called that price tag “an early estimate.”

The city expects to contribute $177 million, on top of $200 million already being spent by the Administration for Children’s Services. The remaining $700 million would come from state and federal sources.

Over the past two years, the city has enrolled at least 50,000 additional students in pre-K programs for 4-year-olds, bringing the total to more than 70,000. Still, research has shown the city’s program is highly segregated — a reality schools Chancellor Carmen Fariña has described as a product of parent choice.

Given the uncertain political climate, officials cautioned the roll-out would probably be even more difficult than expanding pre-K to 4-year-olds. The city will need more classroom space and 4,500 new teachers to staff the effort.

“This is going to be a game-changer,” the mayor said, “but it’s also going to be hard to do.”

You can read the city’s announcement here.

Chalkbeat reporters Alex Zimmerman and Monica Disare contributed to this report.