The owners of two Upper West Side single-room occupancy buildings that also serve as hotels filed suit yesterday to block an anti-flophouse law set to go into effect May 1.

The Manhattan federal court filing says the state regulation — which bans apartment rentals of less than 30 days — will cause “significant economic harm” to the Dexter House at 345 W. 86th St., and the Hotel Alexander at 308 W. 94th St.

Both buildings offer short-term rentals to tourists and other city visitors who can’t afford “traditional luxury hotels,” the suit says.