"Austria and France are the two countries that are losing their triple-A," the source said, meaning that the four other states with top ratings -- Germany, the Netherlands, Finland and Luxembourg -- retained their stellar standing.
Another European source said Austria's downgrade is worrisome for the country's banking sector, which is already considered to be in a fragile state.
The S&P decisions, which the ratings agency was expected to officially announce later Friday, could also have a negative impact on the eurozone's debt bailout fund, which relies on the credibility of the six top-rated nations.
A downgrade of the European Financial Stability Facility would likely increase its borrowing costs, making it more expensive for the EFSF to raise funds on the markets used to provide bailouts.