According to Bank of America Merrill Lynch.
In a note to clients, the US investment bank highlighted the economic benefits of the customs union and single market, as well as the costs of leaving.
Free trade via the customs union with the EU "has been a key driver of rising average living standards in recent decades (though of course there is much debate about the distribution of income)," analysts led by global economist Ethan S. Harris said in the note. "So cutting trade with the EU would be economically negative in our view."
Meanwhile, the single market "helps economies of scale," the analysts said. "The UK attracts foreign direct investment from abroad in part because of its membership, which often boosts productivity by bringing in external expertise."
"So exiting both the customs union and the single market would therefore be economically costly: it would introduce frictions to UK/EU trade that do not currently exist, which means reduced trade volumes between the UK and EU and therefore lower incomes all round."