Moody's Investors Service changed its outlook on the Aaa rating of the European Union from stable to negative late Monday, warning it might downgrade the whole bloc if it decides to cut the ratings on the EU's four biggest budget backers: Germany, France, UK and Netherlands.

The actual rating was kept at Aaa, however.

The move will add to pressure on the European Central Bank to provide details of a new debt-buying scheme to help deeply indebted euro zone states at its policy meeting on Thursday.

Moody's changed its outlook for Germany, the Netherlands and Luxembourg -- some of the continent's best economies -- to negative in July as fallout from thedebt crisis cast a shadow over its top-rated countries. The outlook on France and the UK are also negative.

"The negative outlook on the EU's long-term ratings reflects the negative outlook on the Aaa ratings of the member states with large contributions to the EU budget: Germany, France, the UK and the Netherlands, which together account for around 45 percent of the EU's budget revenue," the ratings agency said, reported Reuters.

Moody's added that it needed to adjust the outlook on the broader EU due to "the likelihood that the large Aaa-rated member states would likely not prioritize their commitment to backstop the EU debt obligations over servicing their own debt obligations," as MarketWatch noted 

Moody's said the EU's rating would be particularly sensitive to any changes in the ratings of these four Aaa member states, implying that if it downgraded these four it might also cut the EU's rating.

Likewise, Moody's said the outlook for the EU could go back to stable if the outlooks on the four key Aaa countries also returned to stable.

The agency also changed to negative the outlook the European Atomic Energy Community (Euratom), on whose behalf the European Commission is also empowered to borrow.

(Reporting by Wayne Cole; Editing by John Mair)

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