The head of Spain’s biggest bank has made an impassioned plea for Europe to adopt a so-called union of its eurozone banks as an “essential” route out of the region’s crisis.

Writing in the Financial Times, Emilio Botín, the 78-year-old chairman of Santanderwho routinely keeps a low profile, says: “Banking union is an ambitious, complex and difficult process?.?.?.?but we cannot afford to postpone it.”

Banking union, hailed by EU leaders in June as an important antidote to the eurozone crisis, would involve pooling supervision of the region’s banks and creating a pan-eurozone safety net to support them. Policy makers believe that the idea, if implemented?promptly?and?effectively, would break the downward spiral created when weak states underwrite weak banks.