Standard and Poor's placed the European Union on watch for a downgrade of its AAA credit rating, a day before EU leaders meet in Brussels in a make-or-break bid to end the region's debt crisis.

Although the crisis is focused on the 17 nation eurozone, S&P said the eurozone countries, especially Germany and France, provided the lion's share of the revenues of the 27-member European Union.

"Given the EU's dependency on such revenues from national budgets, and our recent CreditWatch placements on the 'AAA' ratings on Germany and France, among others, we will concurrently review the 'AAA' long-term rating on the EU with the ratings on the eurozone member states."