European Union finance ministers reached a landmark deal early Thursday that would bring many of the continent's banks under a single supervisor, in what governments hope will be a major step toward resolving their three-year-old debt crisis.

Ministers said the European Central Bank would start policing the most important and vulnerable banks in the eurozone and other countries that choose to join the new supervisory regime next year. Once it takes over, the ECB will be able to force banks to raise their capital buffers and even shut down unsafe lenders.