The situation in the euro zone has shown the difficulties of operating a single currency across several countries, Federal Reserve Chairman Ben Bernanke said on Monday.

"There is a basic question - what is the right size for a single monetary policy?" Bernanke said in response to a question at a panel discussion in London.

While some aspects of the euro zone were optimal, including the credibility of its central bank to deliver low inflation, there were also differences across countries in terms of competitiveness and their different stages in the economic cycle, he said.

"So I think it is a mixed picture," Bernanke said.

(Reporting by Marius Zaharia and Marc Jones, writing by William Schomberg; editing by Ron Askew)