"People are preferring to get their operations done before the referendum rather than after," Chief Executive Laurent Mignon said in an interview, but as the June 23 referendum date approaches "this will be a factor that in my opinion will gradually destabilise markets over the coming weeks."

Should Britons vote to leave the European Union, "I think that would generate a wait-and-see attitude among our clients for quite a while, and there would be nothing anyone could do about that," Mignon told Reuters after quarterly results from the investment banking arm of the mutually-owned banking group BPCE.

He called such an outcome, which recent opinion polls show is a possibility, "a leap into the unknown."

"Markets have a horror of uncertainty," he said. "If we ever were to enter into a Brexit scenario that would be bound to have an impact on market volatility, that would affect the whole of the financial sector and beyond."

He said Natixis would itself be seeking to limit the damage from any vote to leave the EU.

"We'll make sure that none of our activities are vulnerable to a sharp move in sterling, for example," he said.

(Reporting by Julien Ponthus and Maya Nikolaeva; Editing by Andrew Callus and Alexandra Hudson)