City Hall is quietly opening a new mega shelter in Midtown that can hold some 600 migrants from the southern border — even as the flow of buses has slowed to a trickle, city officials said Tuesday.

The new facility at the Stewart Hotel on Seventh Avenue will be run by the city’s Health and Hospitals Corporation and Office of Emergency Management.

It will replace the 300-room shelter and migrant welcome center that is already in the building and being operated by the Department of Homeless Services.  It’s slated to open sometime between Dec. 5 and Dec. 11, according to an internal email obtained by the post

Current residents will be given a choice to remain at the hotel or move to a new shelter run by DHS.

This new facility will be the fourth mega-shelter opened inside a hotel to house the recent arrivals from the southern border, many of whom are seeking refugee status after fleeing the violence and chaos in Venezuela.

The other three emergency sites — known as HERRCs — are located at The Row NYC Hotel, The Watson Hotel and The Wolcott Hotel.

Details about the expected cost of the operation were not immediately available.