BRUSSELS — French President Francois Hollande and German opposition leader Peer Steinbrueck criticized British Prime Minister David Cameron’s proposal to hold a referendum on European Union (EU) membership.

Cameron’s plan, announced last month, is to begin a renegotiation of Britain’s EU membership, seeking to repatriate unspecified powers, if he wins the 2015 general election. He would then hold a referendum on the results by the end of 2017.

Speaking to the European Parliament Tuesday in Strasbourg, France, Hollande said Europe cannot be reduced to a market, a currency, or a set of rules or treaties.

He ruled out each country helping itself to European policies on an “a la carte” basis. Steinbrueck, who heads Germany’s Social Democratic Party, said later he agreed with that view.

“This is a very risky strategy,” he told journalists in London after a meeting with U.K. opposition leader Ed Miliband. “It will cause a lot of uncertainties. It’s very important to have a heavyweight like the UK. in the EU.”

Hollande pleaded for a “differentiated Europe” in which smaller groups of countries unite around specific policies.

He gave the examples of the euro, the passport-free travel zone and a proposed financial-transaction tax — three things Britain isn’t taking part in.