Homeless single adults will have to prove they have no place to go in order to stay in a city shelter starting next Monday.

The decision to implement the policy – which is already in place for homeless families – came after it became apparent that some adults seeking shelter had alternate options such as staying with family or friends rather than sleeping on the streets, the Department of Homeless Services said.

"Over 60 percent of the people coming into the shelter system now are coming from having lived somewhere else,” said Homeless Services Commissioner Seth Diamond. “Less than 15  percent are coming from living on the street."

The city said the mentally ill will be treated differently but there will be a thorough review of everyone else: "We'll be contacting the people that they either lived with or their prior landlords," Diamond said.

Critics of the policy say it results in needy people being wrongly denied shelter.

"This policy is an irresponsible 'no room at the inn' approach that does nothing to address the record number of people experiencing homelessness in New York City as winter approaches," Council Speaker Christine Quinn and council woman Annabel Palma, who chairs the committee that oversees homeless services, said in a joint statement.

Palma said she'd hold a hearing on the issue next week.

Legal Aid Attorney Steve Banks said the policy violates a 30-year-old court order that requires the city to provide shelter to homeless women and men that meet certain criteria.

"It's certain that very vulnerable women and men will end up being denied shelter and on the streets of the city, and that's in nobody's interest whether or not your homeless or not,” Banks said.