In an open letter, 60 lawmakers, economists and business chiefs accused the EU of being "intransigent" in divorce talks and said Britain should threaten to withhold the 39 billion pound ($52 billion) divorce bill it has already agreed to pay.
The letter released Sunday by Economists for Free Trade was signed by prominent supporters of a "hard Brexit," including ex-U.K. Treasury chief Nigel Lawson, Conservative lawmakers John Redwood and Peter Bone, and Tim Martin, chairman of the Wetherspoons pub chain.
They urged U.K. authorities "to accelerate their preparations for 'no deal' and a move to a World Trade Deal under WTO rules."
That would mean tariffs and other trade barriers between Britain and the EU, and many businesses say it would severely harm the U.K. economy. Airbus, Siemens and BMW have all warned recently that leaving the EU without a free-trade deal would hurt British businesses and cost jobs. Airbus alone employs nearly 14,000 workers in the U.K.
Health Secretary Jeremy Hunt said the warnings from businesses were "inappropriate" and undermined chances of getting a "clean Brexit."
"The more that we undermine Theresa May, the more likely we are to end up with 'a fudge,' which would be an absolute disaster for everyone," he told the BBC.
May's Conservative government is divided between Brexit-backing ministers calling for a clean break so that Britain can strike new trade deals around the world, and those who want to stay closely aligned to the EU, Britain's biggest trading partner.
Hunt urged people to unite behind the prime minister, saying she would mix "cautious pragmatism" with a determination to fulfil voters' decision to leave the EU.
On Saturday, however, tens of thousands of anti-Brexit protesters marched in London to demand a new referendum on leaving the EU as Britain marked the second anniversary of its 2016 vote to quit the bloc.
"Brexit is not a done deal. Brexit is not inevitable. Brexit can be stopped," Liberal Democrat leader Vince Cable told the crowd.
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