NEW YORK - Rating agency Moody's Investors Service on Monday changed its outlook on the European Union's long- term debt ratings to negative from stable, sending a warning signal on a possible downgrade to the regional bloc's Aaa rating.

"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 agency said in a statement.

Moody's has previously put all the four countries on negative outlook.

The agency said that the EU's rating would be particularly sensitive to any changes in the ratings of these four Aaa-level member states, indicating that if it downgrades these four countries, it may also cut the EU's rating.

The Moody's move came ahead of a widely-anticipated European Central Bank (ECB) meeting scheduled for Thursday, and is expected to add pressure to the ECB about its new plan to tackle Europe's debt crisis.