WASHINGTON — The Congressional Budget Office said Tuesday that the Supreme Court decision on President Obama’s health care overhaul would probably lead to an increase in the number of uninsured and a modest reduction in the cost to the federal government when compared with estimates before the court ruling.

Of the 33 million people who had been expected to gain coverage under the law, 3 million fewer are now predicted to get insurance, the budget office said in assessing the likely effects of the court decision.

The court said, in effect, that a large expansion of Medicaid envisioned under the 2010 law was a state option, not a requirement.

While it is not yet clear how many states will ultimately opt out of the expansion, the budget office said it now predicted that six million fewer people would be insured by Medicaid, the federal-state program for low-income people. Half of them, it said, will probably gain private insurance coverage through health insurance exchanges to be established in all states.