Gordon Brown has warned that France and Italy may need to be bailed out like Spain because of the unfolding eurozone crisis.
The former UK prime minister said it was not only Greece, Portugal and Ireland facing the threat of euro exit.
But, he added, some of the EU's largest countries such as France, Spain and Italy were also at risk.
Mr Brown also warned that Germany might have to seek additional funding to safeguard its banks.
In a stark warning ahead of next week's G20 summit in Los Cabos, Mexico, Mr Brown said: "The euro area is finally approaching its own day of reckoning."
The former PM said, in a blog for Reuters , that "we are in a downward spiral that shows no sign of ending."
He accused EU leaders of adopting "well-meaning half measures."
"If there is a failure of global leadership next week [at the G20], not only will Europe be condemned to a lost decade but the whole world will pay a fearful price."
The G20 summit is a meeting of 19 countries plus the European Union, which between them account for 80% of global gross domestic product and 80% of global trade.
As host, Mexico's President Felipe Calderon wants the summit to concentrate on how to reduce food price volatility, promote green growth, and boost investment in scientific and agricultural technology.
But in truth, the eurozone crisis is likely to overshadow proceedings, with all eyes on Germany as the potential answer to the region's debt crisis.
Germany 'not immune'
Mr Brown accused eurozone leaders of seeking to "wish away" Spain's banking problems by providing only 100 billion euros of credit when their banks had two trillion euros of liabilities.
Mr Brown also predicted that Italy and France may have to follow Spain in seeking a rescue package for their banks.
"Even German banks," he said, "which are some of the most highly leveraged, are not immune from needing more capital."
Mr Brown said "the final showdown" could be postponed but eurozone leaders had to accept "the inescapable logic" of a fiscal union.