Britain would face gruelling negotiations to set the terms of its World Trade Organization (WTO) membership if it decided to leave the European Union (EU), the group's chief warned.
WTO director-general Roberto Azevedo doubted claims made by supporters of a so-called Brexit that Britain would be able to access the European market through the WTO if it were unable to secure favourable alternative deals on leaving the EU.
"Pretty much all of the UK's (global) trade would somehow have to be negotiated," he told the Financial Times, adding that Britain would not be allowed to "cut and paste" a new deal.
The warning follows similar interventions by the British Treasury, the Bank of England, the International Monetary Fund and US President Barack Obama, who have all raised the spectre of negative economic consequences of a Brexit.
Polls currently show the "Remain" camp in the lead ahead of the June 23 in-out referendum.
Britain's current WTO membership is under the agency of the EU.