U.K. Prime Minister Boris Johnson may have promised a "tariff-free" post-Brexit trade deal at the end of last year, but British businesses are increasingly sounding the alarm over the scale of potential new duties.

Throughout nearly five years of Brexit talks, economists and trade experts repeatedly warned that, even in the best-case scenario, an EU-U.K. accord would only prevent the imposition of tariffs on goods that were predominantly produced in the U.K. Many other goods passing through Britain or distributed from there could face import duties, they warned, but the subject gained only scant attention in mainstream political debate.

In the last couple of days, however, the message has sunk in and a growing number of companies have spoken out over the disturbance that new tariffs could cause for exporters.