If Britain stays in a customs union it must agree to apply the same tariffs on imports from outside the EU and can not negotiate free trade deals with other nations, as the government currently hopes to do once the U.K. leaves the bloc. (cnbc.com)
Even the US, Britain's closest ally, recently got in on the act, warning London that if it leaves the EU, it would «be subject to the same tariffs, and other trade - related measures, as China, or Brazil or India.» (businessinsider.com)
In December 2016, Trump stressed that rather than applying the same tariffs to all companies, he would use punitive tariffs to punish some companies that source overseas. (macleans.ca)