The U.K. government has asked business groups to map their supply chains to flag the areas of the economy most at risk if Brexit imposes additional trading costs on exporters, two people familiar with the matter said.

One of the people expressed bafflement that it's taken so long for the government to ask for the data. It's just 10 months until the country formally quits the European Union, and Prime Minister Theresa May's Cabinet—and, indeed, her Conservative Party—is still procrastinating on what kind of customs arrangements to pursue post-divorce.

The course the government decides on will determine what burden exporters face, and whether it's still viable for them to trade with the EU and with third countries Britain now has trade agreements with as a member of the bloc.