HSBC Holdings said some of its largest clients have already asked for their business to be routed through the bank's offices in mainland Europe and aren't waiting to see what Brexit deal the U.K. hammers out with the continent's trading bloc.

“A small number of our larger clients are asking us to book more of their trade and foreign-exchange activity in their French operation through our Paris office than their U.K.” divisions, Noel Quinn, head of global commercial banking, said in an interview. Executives at multinational companies are “making plans to ensure they can continue to trade irrespective of the outcome. They can't afford to wait for a decision that may not emerge until two years' time.”

Global banks have started arranging for some British-based operations to move to new or expanded offices inside the EU after British Prime Minister Theresa May triggered discussions to leave the trading bloc. Privately, many executives at the world's biggest firms say they're now assuming the result will be a “hard Brexit” — the loss of their right to sell services freely around the region from the U.K. That means they have to put contingency plans in place before the end of the two-year negotiation period.