A lawyer sneaked a former divorce client into an illegal sweetheart lease in an Upper East Side building and never told the incoming landlord, according to a lawsuit.

The Yashar Foundation says it wasn’t told that 179 E. 94th St., a four-story building with one apartment and 19 single-occupancy rooms, had a tenant during months of negotiations to lease the space.

Yashar, a Brooklyn nonprofit that funnels cash to other charities, took over the space — a four-story building with one apartment and 19 single-occupancy rooms — May 1 and allegedly discovered Sonia Hassan had a 15-year, $600-a-month lease for room 203, according to a Manhattan Supreme Court lawsuit.

Brian M. Limmer, the lawyer who represented the building’s prior owner, “also acted as Hassan’s divorce attorney,” the foundation charges. Limmer and Hassan did not return messages seeking comment.

The lease “was put into place collusively,” and was not recorded with the Division of Housing and Community Renewal, which oversaw the building, or the City Register, according to Yashar, which wants the leaseit voided. Yashar is seeking unspecified damages, and wants Hassan’s lease voided and her booted from the building.

“The lease provides the tenant with a right to sublet, assign, or permit any other person to use the premises,” Yashar claims.