Billions of euros in aid given by the Dutch authorities to Fortis’s Dutch banking unit and its recently-acquired ABN Amro operation may have amounted to cheap funding and been illegal under EU rules, Brussels warned Wednesday as it opened a probe into the matter.

In the toughest language to date over state aid to a crisis-hit bank, the European Commission said it doubted that the way the Dutch government bought out Fortis Bank Netherlands from its ailing Fortis Bank parent, in an emergency rescue last October, complied with state aid rules.