Credit ratings are a key part of the financial system because investors use them to judge how likely they are to get their money back. But the financial crisis led to unease that the market is relying on them too much.
The European Securities and Markets Authority (ESMA) published on Monday results of its investigation into how Moodys, Standard & Poors and Fitch compiled ratings on sovereign bonds between February and October this year.
It criticised delays in the publication of ratings changes and poor confidentiality controls at the agencies.
Sovereign ratings became politically charged at the height of the euro zone crisis when S&P infuriated Greece in 2011 by cutting the rating of its debt while the country's EU bailout was being renegotiated.
This led to the third of three EU laws to regulate rating agencies in as many years. From next month, they can only release changes to sovereign ratings according to a pre-set calendar to improve transparency.