Developer Millennium Partners is asking to reduce the residential portion of its Winthrop Center skyscraper under construction in Boston’s Financial District, citing impacts from COVID-19 and a downturn in condo markets and capital sources.

The developer notified Boston Planning & Development Agency it will shrink the residential portion from 387 to 321 units and market them as apartments, rather than condos as originally envisioned. The office portion of the project remains notably unchanged at 772,000 square feet.

Millennium bought the 1-acre site in 2018 for $102 million but agreed to pay the city up to an additional $64 million as a percentage of the proceeds from condo sales. But in October, Millennium announced that it was reducing the condo portion from 500 to 387 units.

BPDA spokeswoman Bonnie McGilpin said today that Millennium remains responsible for the full payment, and “the BPDA and City are currently in discussions about the structure of the remaining payment.”

In a statement this afternoon, BPDA Director Brian Golden said the BPDA is hiring an outside consultant to evaluate the market for high-rise condo financing.

“The approval of Millennium’s proposed changes will not go forward until that review and a public process alongside the community is complete,” Golden said.

The redesign eliminates the upper floors of the east tower on the T-shaped site at 115 Federal St. along with a glass-enclosed sky bridge connecting the two towers.

The notice of project change reduces the residential portion from 663,000 to 583,000 square feet.