A G-20 summit this week in France should send a "strong signal" that Europe will follow through on plans to overcome its sovereign debt crisis, World Bank President Robert Zoellick said on Tuesday.

Speaking to reporters ahead of the G-20 leaders' meeting in Cannes, France, Zoellick said Greece's call for a referendum on a new Greek bailout plan adds to market uncertainty and if it failed "it would be a mess."

"The euro zone deal has bought some time and the challenge is how to use the time," said Zoellick. "It would be very useful if the G-20 leaders can send a strong signal on follow through after the euro zone announcement so as to sustain and build confidence," he said.