Standard & Poor’s told Fairfax and Prince William counties that their coveted AAA credit ratings will remain intact despite the unprecedented downgrading of federal debt last week.

Sharon Bulova, chairwoman of the Fairfax Board of Supervisors, said she was relieved to learn late Tuesday from Standard & Poors that it is unlikely to change the county’s rating.

Steve Solomon, Prince William’s finance director, said S&P told the county Monday that the AAA rating would remain.

Moody’s confirmed both counties’ AAA ratings last week, but put it on a negative watch list along with six other Northern Virginia jurisdictions.

The Northern Virginia localities will meet with Moody’s in the next 90 days to make a case as to why they should be off the list.

A downgrade would make it more expensive for counties to borrow money for roads, schools, hospitals and other projects.

The Washington area is particularly vulnerable because it has a high concentration of federal employees and government contractors.

The localities had been on a watchlist for more than a week before Congress passed a deal to raise the debt ceiling.

Virginia had 15 municipal authorities on the list, the highest of any state. The Virginia municipalities under review include Alexandria, Fairfax City, Herndon and Vienna, the Fairfax County Water Authority, and Arlington, Fairfax, Loudoun and Prince William counties.