Philipp Hildebrand, a former governor of Switzerland's central bank and now vice chairman of BlackRock, told a City conference that prospects for financial services in Britain would be severely undermined if the June 23 referendum on EU membership resulted in a majority for Brexit.

He said that non-EU Switzerland spent 10 years negotiating 120 trade pacts with the EU that do not include financial services, even though the Alpine state must accept, as part of those deals, the free movement of EU citizens and primacy of EU court rulings.

The notion that a Britain outside the EU could navigate a trade deal free of these two constraints is simply an illusion, and for London not to have a financial services agreement with the EU would be ruinous, Hildebrand said.

"When you look at the facts, the overwhelming conclusion is that it would be incredibly irresponsible to attempt this on your own."

(Reporting by Huw Jones; Editing by Mark Heinrich)