Mayor de Blasio sounded a doomsday alarm on Friday over the slim possibility that lawmakers would allow rent regulations to expire for 1 million city apartments.

Warning that failing to strengthen the protections would also cost the city tens of thousands of affordable-housing units, Hizzoner invoked a “nightmare” scenario if the law is allowed to sunset Monday.

“If Albany does literally nothing, then there’s a real nightmare coming where the rent-regulated units we have today . . . will no longer be affordable,” he told thousands in an AARP telephone town hall.

Representatives for landlords said the mayor was needlessly stoking fear.