South Korea’s Fair Trade Commission has raided the local offices of the carmakers Mercedes-Benz, BMW, Lexus and Audi, in a surprise probe that could complicate the progress of bilateral trade agreements with the US and European Union.

The raids are likely to give American opponents of the Korea-US trade agreement, signed in June, additional leverage, and make ratification by the US Congress more difficult. The deal, which could boost bilateral trade to almost $100bn (€68bn, £49bn) a year, has been strongly criticised by US carmakers, who say it is lopsided in Korea’s favour.