Ratings agency Standard and Poor's (S&P) said France and Germany are among 15 nations that have been put on "credit watch" due to fears over the impact of the debt crisis.
The move means six countries with the top AAA ratings - including France and Germany - would have a 50% chance of seeing their ratings downgraded.
Austria, the Netherlands, Finland and Luxembourg also currently have top AAA rating.
S&P said it wanted to see speedy action to resolve the debt crisis.
It is quoted in the Telegraph as telling member nations the "lack of progress European policymakers have so far made in controlling the spread of the financial crisis may reflect structural weaknesses in the decision-making process within the eurozone and European Union".
Stocks fell on the announcement, reversing much of the gains made earlier on Monday as investors were buoyed by news of a Franco-German plan to ease the eurozone's debt burden.
At meetings in Paris ahead of an EU summit on Friday, German Chancellor Angela Merkel and French President Nicolas Sarkozy discussed a new European treaty which would create a "golden rule" of balanced budgets for eurozone states.
Members would face automatic sanctions for large deficits.
As well as rallying stock markets, the announcement saw the costs of borrowing for Italy and Spain fall by half a percentage point and the euro strengthen against the dollar.
But all indicators fell back following news of S&P's threat. The Dow Jones closed up 78 points, giving up much of a 167-point gain it had made earlier in the day.
The main points of the new treaty:
- Automatic sanctions for breaching deficit ceilings of 3% of GDP and a requirement for balanced budgets.
- Speeding up implementation of the permanent bailout funds, the European Stability Mechanism, to 2012, with the introduction of qualified majority - 85% - for decisions, instead of unanimity.
- No more haircuts for bondholders.
- A monthly meeting of eurozone leaders until crisis ends, focusing on growth in Europe.
- ECB's role to remain unchanged - will not be lender of last resort - and there will be no eurobonds.
'Right thing to do'
£69m spent on upgrades
European fintech market 'underserved'