In the wake of the global financial crisis, Europe changed the rules to ensure that the collapse of a single bank could not threaten the region’s economy. That new system will now be put to the test.

Banco Santander, Spain’s largest bank, said on Wednesday that it would rescue a troubled rival, paying a symbolic amount of one euro for Banco Popular. Santander will also raise an additional €7 billion, or about $7.9 billion, to bolster Banco Popular’s balance sheet.

The rescue is the first in which the European Central Bank has determined that a eurozone lender has failed or is about to fail. The central bank assumed authority over eurozone banks in 2014 as part of changes intended to prevent banking crises like that of 2008.

But the failure of Banco Popular was nevertheless a reminder that, a decade after the financial crisis, the eurozone’s banking system remains vulnerable.

Many banks in Italy, Germany and elsewhere in the 19-country euro area suffer from piles of bad loans, thin capital cushions and meager profits. The woes of banks continue to weigh on economic growth in the bloc.

The decision by the European Central Bank to classify Banco Popular as a failed or nearly failed bank could also raise questions about whether the central bank is applying new regulations consistently. The Spanish bank’s takeover comes less than a week after the European Commission gave preliminary approval to plans for a government rescue of Monte dei Paschi di Siena, a troubled Italian bank.

For Monte dei Paschi to qualify for government aid, the European Central Bank must determine that it is solvent. But the Italian lender fared significantly worse than Banco Popular in assessments last year of bank health. The central bank could be accused of playing favorites if it clears the way for Monte dei Paschi to receive a taxpayer rescue.

The Monte dei Paschi case has significant political dimensions. Without government aid, the Italian bank would be forced to impose losses on thousands of middle-class savers who bought the bank’s bonds. There is fear the public backlash would benefit populist parties and add to political turmoil in Italy.

In the case of Banco Popular, the European Central Bank said that it had determined on Tuesday that the lender was “failing or likely to fail.”

“The significant deterioration of the liquidity situation of the bank in recent days led to a determination that the entity would have, in the near future, been unable to pay its debts or other liabilities as they fell due,” the central bank said.

Shares of Banco Popular have dropped by more than half in the past week over concerns that the bank would need additional capital as it struggled under the weight of billions of euros in bad real estate loans.

The central bank said it contacted the Single Resolution Board, which is responsible for handling the winding down of banks that fail. The board adopted a plan to sell Banco Popular to Santander.

The combined business of Banco Popular and Santander will be led by the current management team of Santander Spain with Rami Aboukhair as chief executive, Santander said.

“We welcome Banco Popular’s customers as part of the Santander Group and will work hard to continue serving them at the highest standards through the transition and beyond,” Ana Botín, the executive chairwoman of Santander, said in a news release.

“The combination of Santander and Popular strengthens the group’s geographic diversification at a time of improving economic conditions in both Spain and Portugal, and will allow us to continue to deliver for customers and shareholders on all of our commitments,” she added.

To bring Banco Popular’s balance sheet in line, Santander said it would make additional provisions for nonperforming assets of €7.9 billion, including €7.2 billion for real estate.

Santander said it expected to reduce Banco Popular’s real estate exposure significantly, as Santander has done with its own operations in recent years.