Nanny agencies, an often-vital link between New York parents and childcare professionals, are under investigation by a City Council committee seeking to uncover how they treat their nannies, how much they charge parents, and whether they are licensed.

A report detailing the results of the investigation is expected in late April.

The investigation has won support from New York’s most famous former nannies, Nicola Kraus and Emma McLaughlin, authors of “The Nanny Diaries.” Both said they hope it leads to better working conditions for nannies.

Ms. Kraus said she has heard of cases where an agency official tells a family the agency will cover the nanny’s first paycheck and then withhold payment.

“The opportunities for exploitation are many,” Ms. McLaughlin said. “Unfortunately, I think it can happen and does happen.”

A council member of the Upper West Side, Gale Brewer, is a member of the Oversight and Investigations Committee, which is conducting the investigation, said people “don’t realize how few rights they have. There are a lot of abuses.”

She is the author of a 2003 law requiring agencies to provide nannies and parents with information about nannies’ rights and ensure parents sign a document outlining a nanny’s job requirements.

It is common for nannies to be hired to care for children, only to discover that they also are expected to cook, clean, and wash clothes, she said. Ms. Brewer has heard from some nannies that not all agencies are distributing the required information, she said.

Several nanny agencies in New York City were contacted, but none could be reached for comment yesterday.

There are 200,000 domestic workers in New York City, according to a 2006 report by Domestic Workers United and the DataCenter. The term refers to nannies, housekeepers, elderly companions, cleaners, baby sitters, baby nurses, and cooks.