The UNCTAD economists said Britain's total merchandise exports last year stood at around USD 450 billion, and that its exports to other EU countries accounted for about half of that amount.
The report was released as British Prime Minister Boris Johnson lost his working majority in parliament ahead of a showdown with rebel MPs over fears he is steering the country towards a no-deal Brexit next month, which could lead to a snap election within weeks.
"The nearing Brexit deadline, along with increased uncertainty on outcomes, is problematic for UK exporters," UNCTAD said in a statement, urging London to quickly seek policy solutions to provide at least short-term relief to the "Brexit crunch".
The economists said that most of the British losses in the EU market would be concentrated in motor vehicles, which would take a $5-billion hit; meat and other animal products as well as the apparel and textile sector, which are both set to suffer losses of around $2 billion.
At the same time, the report cautioned that British exporters also stand to take a significant hit on trade beyond the EU. Through its EU membership, Britain today is part of around 40 trade agreements offering members of the bloc preferential market access in about 70 countries.
"In the event of a no-deal Brexit, and in the absence of replacement agreements (rollover deals), the UK would abruptly lose preferential access to these markets and, by default, would have to export under World Trade Organization Most Favoured Nation (MFN) tariffs," the report said.
Most of Britain's exports beyond the EU go to countries that do not currently give preferential treatment to the bloc, including China and the United States.
But about 20 per cent of the country's non-EU exports are at risk of facing higher tariffs from countries such as Turkey, South Africa, Canada and Mexico, UNCTAD said, estimating that this could cost the British economy nearly USD 2 billion in exports.
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