As U.K. Prime Minster Theresa May prepares to relaunch her Brexit strategy, global banking bosses meeting nearly 7,000 miles away from London are drawing their own road maps in the event she fails.

"We are all planning for the worst, but hoping for the better," John McFarlane, chairman of told CNBC at the Singapore Summit.

Deutsche Bank CEO John Cryan agreed from the same event: "The more clarity we have, the earlier the better. That goes without saying. But I think, just on a contingency planning basis, it's right to think the worst."

The C-suite is staying tight lipped on the exact number of jobs slated to leave London.

"The truth of the matter is: We don't know. But it's not a big number," McFarlane said, while estimating around a hundred jobs. "The EU is about 10 percent of our total bank and, therefore, if we were moving all of it, then we would have to move the balance sheet. However, if wholesale activities can remain in London, we don't have to move the balance sheet and therefore, we don't have to move too many people," he explained.

Asked whether it was realistic that half of Deutsche Bank's London headcount was slated to move to Frankfurt, as widely reported, Cryan replied, "No I don't think so.

"Those estimates are difficult to gauge because the scenarios can be different," he added. "We're in a happier position because our capital and our bank are already German. So we can designate the London branch to something but only as a booking center."