Europe needs to be reinvented according to a new plan that could be put to an EU-wide referendum later, but it would be "totally irresponsible" to hold a plebiscite in France before that, France's would-be president, Alain Juppe, said on Monday.
Speaking after Britain voted to leave the EU, Juppe also said Britain's exit should be dealt with quickly.
"A referendum will (then) be needed at some stage in Europe's reconstruction, not only in France but all countries concerned," Juppe said.
He joins a growing chorus of French politicians who since the Brexit vote have called for an EU-related referendum in France, among them President Francois Hollande's economy minister, Emmanuel Macron.
But with the exception of the far-right National Front, those calls are to vote on some new form of EU project or treaty, not on membership of the bloc.
In a country that is increasingly disillusioned with Europe, the outcome of a referendum would be hard to predict. In 2005, a proposed new EU constitution was blocked by referendums in France and the Netherlands.
Juppe, prime minister from 1995 to 1997, is campaigning to persuade the conservative Les Republicains party to nominate him rather than ex-president Nicolas Sarkozy in November's primaries. Polls rate him above Sarkozy among right-wing voters, and he has the advantage of appealing more to left-wingers, too.
On the EU's future, Juppe staked out a number of positions. Membership expansion should be halted, he said, and the remit of EU institutions pared back. Euro zone convergence on tax and social matters should be intensified and a new deal struck on control of the EU's external frontiers.
The Schengen accord on freedom of movement for people in the EU has not worked, he said. That raises the question of whether the EU's external frontiers should be policed by a European border control force.
"One thing is certain," said Juppe. "The Europe of tomorrow will clearly operate more than now at variable speed levels. The United Kingdom's departure should allow those who want to forge ahead faster to do so. It's an opportunity we should seize."