"We believe and we hope" that a deal is possible to keep Britain in the EU," Valls told France's parliament.
"Because the departure of Britain would signify ... a shock for Europe, but more importantly a shock to the way the world sees Europe, which is already in crisis."
A summit of all 28 EU leaders will take place in Brussels on Thursday and Friday, at which British Prime Minister David Cameron hopes to win backing for a package of reforms to his country's relationship with the bloc.
Cameron says the reforms are necessary to convince Britons to vote to stay in the EU in a referendum that could take place as soon as June.
Valls said France would be "particularly vigilant" on the issues of free movement of EU citizens, and relations between countries in the eurozone and others.
"Europe should be a place of solidarity between states and we cannot choose 'a la carte' based on what suits us," Valls said.
"Europe is a whole that we can adapt, but we can't rework it only according to our particular interests, otherwise a line is crossed."