This is the latest signal that France's top-tier status is at risk.
An S&P spokesperson in Paris said the agency did not comment on rumours. A spokesman in Melbourne earlier also declined to comment on the report, which if true would signal a heightened risk of a downgrade in the weeks ahead.
"It could happen within a week, perhaps 10 days," La Tribune quoted a diplomatic source as saying of a change to the outlook.
The economic and financial daily said S&P -- which cut Belgium's credit rating to double-A from double-A-plus Friday -- had planned to make its announcement on France the same day but postponed it for unknown reasons.
The euro briefly dipped on the report, which coincided with news that credit rating agency Moody's could downgrade the subordinated debt of a swathe of euro zone banks.
French Finance Minister Francois Baroin said the focus should not be solely on France and that while the euro zone debt crisis was serious, France was "clear-sighted" on it.
"Everyone is concerned, not just France. It's all the euro zone countries," he told France Info radio, asked about the La Tribune report.
"France is not an island or economically cut off from the world. It depends on different parts of the euro zone for a large part of its economic activity and that's why we are, to a large extent, clear-sighted on the crisis."