Mayor Michael Bloomberg has begun charging rent to homeless families who live in publicly run shelters but receive income from jobs.

The new policy is based on a 1997 state law that was not enforced until last week when shelter operators started requiring shelter residents to pay a portion of their income based on factors including family size and the shelter being used, without exceeding 50 percent of a family's income.

City officials said the new rent requirement is in line with the federal Section 8 housing voucher program and has been in development since a 2007 state audit forced them to pay back $2.4 million in state housing aid that should have been covered by homeless families with income.

While it is unclear exactly why the state law was not forced until now, New York City does have an above-average rate of working homeless families than anywhere in the state, as well as higher living costs than almost anywhere else in the country.

Anthony Farmer, a spokesman for the State Office of Temporary and Disability Assistance, said the new policy will eventually affect about 2,000 of the more than 9,000 families in New York City shelters. About 500 families have already been informed they're expected to begin paying rent on May 1, with city officials saying they've started with families that are new to shelters.

But many homeless advocates and shelter residents were not pleased with the new policy, calling unfair and counterproductive.

Patrick Markee, the senior policy analyst of the Coalition for the Homeless, said the new policy is "impractical," adding that, "It's going to make families stay in shelter longer because they'll have fewer financial resources."

President and chief executive of the Partnership for the Homeless Arnold S. Cohen agreed, saying, "We're dealing with the poorest people, the people who are the most in need, and we're asking them to pay for a shelter of last resort. As a city and a state that has a history of social and economic justice, I think we can do better than that."