To try to head off such anxiety, the governor of the Bank of Spain said this week that regulators in Spain had conducted stress tests on all Spanish banks and intended to publish the results.

Two Spanish banks are included in the wider, European stress tests covered by Thursday's deal: Santander and BBVA. Other major European banks covered include Royal Bank of Scotland, BNP Paribas and Deutsche Bank.

Under the agreement there was also a pledge to show flexibility in applying rules limiting government aid to companies. That would allow European governments to move to recapitalize any banks found to be in trouble — before the publication of the stress-test results.

“If state intervention is needed, this will be examined in a timely manner,” Mr. Barroso said.

“I think it’s good news,” said Nicolas Véron, an economist at Bruegel, a research organization in Brussels. “Nations have come to the realization they couldn’t go on telling people everything was fine and giving no evidence.”

Mr. Véron predicted that evaluations of the large banks, along with plans by Spanish authorities to disclose results of their tests of domestic banks, would force other countries to follow suit and disclose the financial state of smaller institutions.

Image From left, José Manuel Barroso, Silvio Berlusconi and Nicolas Sarkozy in Brussels on Thursday. Credit... Benoit Doppagne/BELGA, Via European Pressphoto Agency

“Once you have a benchmark for the first 25 banks, then this will have trigger effects,” he said.

One big risk is that some banks will prove to be weaker than has been publicly known, or even insolvent once forced to do a frank evaluation of their financial state. “It could be there will be some delicate moments in the process,” Mr. Véron said. “But it’s much better than the stonewalling we have had.”