City workers looking forward to moving into spectacular new offices at the World Trade Center in a couple of years had better get a Plan B.

Developer Larry Silverstein surprisingly indicated yesterday that private tenants might take over the nearly 600,000 square feet that the city has committed to renting in the second tower he’s putting up at the WTC.

“It’s totally conceivable that with time, the city’s obligation may not be necessary,” Silverstein said at 7 WTC, where the law firm WilmerHale announced that it’s taking 210,000 square feet.

To secure financing for the nearby Tower 4, the city and the Port Authority agreed five years ago to rent two-thirds of its 1.8 million square feet.

Two months ago, Silverstein exercised that option, which requires the city to move in once the building is completed.

Mayoral spokesman Jason Post said, “If Silverstein Properties found a tenant and asked the city to terminate the lease at 4 WTC, we would agree.”