Speaking to investors as part of a European roadshow, Riley said the collapse of the euro would be disastrous for the global economy, and while it is not Fitch's baseline scenario, it could happen if Italy did not find a way of its debt problems.
"The end of the euro would be cataclysmic. The euro is a reserve currency," Riley said. "What would that do in terms of financial and political stability?"
"It is hard to believe the euro will survive if Italy does not make it through," he said, adding that while many saw Italy as too politically and economically important to be allowed to fail, "one might also argue that it is too big to rescue."
The warning pushed the euro down to within touching distance of a new 16-month low versus the dollar.
Riley urged the European Central Bank to abandon its current reluctance to scaling up its purchases of troubled euro zone debt such as Italy's and drop its resistance to the bloc's bailout fund, the EFSF, borrowing directly from it.