PARIS • For the growing chorus of observers who fear that a breakup of the eurozone might be at hand, German Chancellor Angela Merkel has a pointed rebuke: It's never going to happen.

But some banks are no longer so sure, especially as the sovereign debt crisis threatened to ensnare Germany itself this week, when investors began to question the nation's stature as Europe's main pillar of stability.

On Friday, Standard & Poor's downgraded Belgium's credit standing to AA from AA+, saying it might not be able to cut its towering debt load any time soon. Ratings agencies this week cautioned that France could lose its AAA rating if the crisis grew. On Thursday, agencies lowered the ratings of Portugal and Hungary to junk.

While European leaders still say there is no need to draw up a Plan B, some of the world's biggest banks, and their supervisors, are doing just that.

"We cannot be, and are not, complacent on this front," Andrew Bailey, a regulator at Britain's Financial Services Authority, said this week. "We must not ignore the prospect of a disorderly departure of some countries from the eurozone," he said.

Banks including Merrill Lynch, Barclays Capital and Nomura issued a cascade of reports this week examining the likelihood of a breakup of the eurozone. "The eurozone financial crisis has entered a far more dangerous phase," analysts at Nomura wrote Friday. Unless the European Central Bank steps in to help where politicians have failed, "a euro breakup now appears probable rather than possible," the bank said.

Major British financial institutions, like the Royal Bank of Scotland, are drawing up contingency plans in case the unthinkable veers toward reality, bank supervisors said Thursday. U.S. regulators have been pushing American banks like Citigroup and others to reduce their exposure to the eurozone. In Asia, authorities in Hong Kong have stepped up their monitoring of the international exposure of foreign and local banks in light of the European crisis.

By contrast, banks in big eurozone countries that have only recently been infected by the crisis do not seem to be nearly as flustered.

Banks in France and Italy in particular are not creating backup plans, bankers say, for the simple reason that they have concluded it is impossible for the euro to break up. Although banks like BNP Paribas, Société Générale, UniCredit and others recently dumped tens of billions of euros worth of European sovereign debt, the thinking is that there is little reason to do more.