For many trade specialists and economists, the huge unknown underlying Great Britain's possible departure from the European Union is what impact it would have on the British economy, and what that would mean for Americans who do business there.

It is Andrew Hood's job to advise hedge funds, investment banks, manufacturers, and others on the potential fallout if British voters decide in a June 23 referendum that it is time to leave.

What that means essentially is getting clients ready to get ready.

Hood, the former top legal adviser to British Prime Minister David Cameron, is now a London-based lawyer focusing on international trade and European Union regulation for Dechert L.L.P. of University City.

If British voters step off the edge of a trade precipice, many of Hood's clients want to know how their businesses will be affected. Hood says that it is hard to know with any precision, but it's time to start thinking about it.

"Different people take different attitudes," Hood says. "The big industries are preparing very thoroughly, trying to get a sense of what the impact will be, their supply chains, and their customer base. Others are doing something cursory and a third category is doing nothing."

Because the impact of a British exit is so unknowable, the temptation to do nothing is quite human. But the consequences of being caught flat-footed might be painful.

For those who procrastinate, "I would not want to be chairman of the risk committee or audit committee on the 24th of June, with the United Kingdom wanting to leave and the board asking, 'OK, what is the plan?' " Hood said.

The latest British polling shows voters who express a view are leaning slightly in favor of the U.K. staying. British oddsmakers are giving only a 36 percent chance that British voters will choose to leave. The problem for Hood's clients is that the political relationships with the continent are so complex that it's impossible to predict how the U.K. would engage with the rest of Europe after a departure, or as the media have it, a "Brexit."

One of Hood's specialties is trade agreements, and he tends to focus on worst-case scenarios. Dechert is one of the top financial services law firms in the U.S. and one of the few outside Wall Street that has a big Wall Street practice.

So its practice in London has a healthy mix of hedge funds and investment banks, among others. These clients are busily gaming out whether a Brexit would diminish London's preeminence as a banking capital. Some financial services firms are discussing the possibility of moving operations to the German banking center Frankfurt. But as Hood points out, simply getting thousands of employees whose mother tongue is English newly situated in a foreign land would pose tremendous hurdles.

Staying would pose its own problems, though.

The European Union has a so-called passport system for financial services companies that accepts that a firm, based on regulatory scrutiny in its home country, is qualified to operate throughout the Euro zone. But what happens if British-based firms are denied passports because their regulators no longer are recognized by authorities in Brussels?

Similarly vexing is the fate of the European employment market. Great Britain now has the most robust economy for jobs in Europe, and the free movement across borders has meant that U.K.-based employers have access to a steady stream of workers from the continent. What is not known is whether this ready supply of workers would be shut down.

Even if the Europeans seek to preserve current trading relationships after an exit, treaties would have to be negotiated on myriad issues with the remaining 27 members of the European Union, Hood says. Leaving aside the question of political resistance, overcoming bureaucratic inertia - sometimes it is difficult to get the right person on the phone - would be formidable by itself, Hood said.

Hood says the fundamentally cautious nature of the British electorate will prevail June 23, and voters will choose to stay.

But the consequences are potentially so enormous that it is time to start planning, no matter what the oddsmakers say.