British exporters would face an extra £5.6 billion ($8.2 billion, 7.2 billion euros) of annual customs duties if the country votes to leave the EU, the World Trade Organization (WTO) chief will warn on Tuesday.

Britain would have to negotiate new trade deals with the EU and with each of the 58 countries that have free-trade deals with the bloc, Director General Roberto Azevedo was to say at a conference in London, according to a copy of the speech obtained by AFP.