Dozens of the European Union’s trading partners from South Africa to South Korea could impose tariffs on British exports next year, experts have warned.

About 70 countries holding EU free-trade agreements may refuse to recognise the UK as a member state from next March, according to lawyers.

The EU pledged to “notify” trading partners that Britain should be treated as a member during the 21-month transition period after it leaves the EU, but it cannot force such countries to do so.

As Britain will remain inside the EU customs union until December 2020, it is obliged to maintain duty-free trade on imports from the EU’s free-trade agreement partners. City lawyers believe that the same countries may decline to treat Britain as an EU member after