Emmanuel Macron said a Brexit could also result in an exodus of London's financial workers.
The former party leader says a Leave vote would strengthen Britain's hand.
After the concessions Cameron secured, the temptation for populist leaders to blackmail the EU will be hard to resist.
The former allies clashed in Britain's House of Commons on Monday.
David Cameron seeks to rally his divided Conservative Party behind a deal to remain in the bloc.
Agreement fires starting gun on British Membership Referendum
David Cameron is still hammering out a deal
Will the U.K. pull out at the crucial moment?
David Cameron's center-right party shares a European parliament group with anti-immigrant populists.
Just 22 percent of voters agree that the draft membership deal is a good one.
For those who want Britain to stay in, the battle is now on.
The EU has proved once again why it is an affront to democracy, says a leading Eurosceptic.
Proposed terms on EU membership are to be debated by national leaders ahead of a U.K. referendum.
European Council President Donald Tusk will publish a draft proposal for reforms on Tuesday.