Some of Fairfax County’s most vulnerable people, and the nonprofit organizations and charities that serve them, fear that a key social services agency’s $8 million budget deficit could jeopardize programs for at-risk children and people with mental illness, intellectual disabilities and substance abuse problems.

The widening shortfall at the Fairfax-Falls Church Community Services Board, which provides services to thousands of people, has forced the publicly funded agency to freeze hiring and make some modest cuts to programs. But with the deficit expected to grow next year, it is considering more drastic measures, including reducing staff, closing treatment centers and a further scaling back of services.

In March, the agency announced cuts to Infant & Toddler Connection, which provides services for newborns and young children who show signs of intellectual delay in development. For the first time in the agency’s history, children are being put on a waiting list, which means that although the program provides assessments of children, they face delays receiving critical therapies, officials said.

The agency, which receives most of its funding from the county, might also close New Horizons, a 16-bed treatment facility for adults with mental illness and substance abuse problems in Mount Vernon, and ­Sojourn House, an eight-bed residential program for mentally ill girls.

Advocates for the disabled have expressed alarm about proposed reductions to Employment and Day Services, which aids young people with intellectual disabilities after their graduation from Fairfax public schools. Those cutbacks could force more than 60 families onto a waiting list. Agency officials have asked the county to find money to fund programs for 19 young people whose needs are most acute.

“These are vulnerable people in dire need of services,” said Rikki Epstein, executive director for the Arc of Northern Virginia, a nonprofit organization that assists people with intellectual disabilities.

Kymberly S. DeLoatche, executive director of the Autism Society of Northern Virginia, said she is worried that the cuts could force some parents or family members to quit their jobs to provide care that their children would otherwise receive from professionals.

“So that is a huge gaping hole,” she said. “And our families are already on the edge of that hole.”

The agency’s budget problems have prompted heightened scrutiny from the Board of Supervisors, which has had to find more money for the agency at a time when the county’s finances are still feeling the aftershocks of the 2007-09 recession. On Tuesday, the supervisors are to review a plan proposed by the Fairfax-Falls Church Community Services Board to address the budget gap. The county also plans to hold a series of public hearings, beginning June 4, to gauge opinion about how to handle the problem.

The Community Services Board administers social services programs for more than 22,600 people. Much of the work is done under contract by private organizations, but the CSB oversees the services and finances them with federal, state and local funding.

In fiscal 2011, the CSB’s expenditures reached nearly $155 million, up from about $139 million the previous year. The county’s contribution, which is about 70 percent of the CSB’s budget, is expected to rise to $100.4 million from $95.7 million.

In the current fiscal year, the CSB has a deficit of about $8 million, which is expected to deepen to nearly $9.5 million in the next fiscal year, which begins July 1. The red ink forced the Board of Supervisors to establish a $4.2 million reserve fund for the anticipated shortfall.

George Braunstein, the CSB’s executive director, blamed the deficits on a rising need for social services after the recession and inadequate revenues to pay for them. For example, overall clientele has risen about 5 percent a year, while the demand for Infant & Toddler Connection jumped 38 percent in the past two years, agency officials said.

The county has slashed general-fund spending for the agency by $15 million over the past four years, according to a memo Braunstein sent March 1 to agency personnel. His memo said that higher personnel costs — “primarily due to cost of increased fringe benefits” — also bit into the agency’s budget.

In an interview, Allan Phillips, director of Infant & Toddler Connection, expressed frustration that the federal and state governments have mandated the program but failed to consistently provide adequate funding. He said early intervention is more cost-effective than a lifetime of intense care.

“This is one of the best programs government ever came up with,” Phillips said. “There’s no one I know that thinks this is a wasteful program.”

Glenn Kamber, a CSB member representing Hunter Mill, said funding problems are not new for the agency, because political leaders at all levels have a tendency to ignore its funding needs until those needs become critical.

“The only time they really listen is when the waiting lists start to increase,” Kamber said. “In addition to all this, the state has put new populations and new responsibilities with the CSB without giving more funds to handle them. That’s our annual dilemma.”