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Theresa May should put together to exit the EU with no deal to have "real leverage" in Brexit negotiations, a letter from 60 politicians and enterprise figures says.
The prime minister also needs to reserve the precise to "take with it the £39bn it has offered to pay as part of a divorce settlement", it says.
Signatories urge the federal government to speed up plans to function below World Trade Organization (WTO) guidelines.
Downing Street mentioned it was assured the UK would get a whole lot.
Former cupboard ministers, economists and enterprise figures together with former chancellor Nigel Lawson, vocal Brexiteer John Redwood, and Wetherspoon chairman Tim Martin signed the letter.
It was organised by Economists for Free Trade (EFT) and asks Mrs May to warn the EU that regardless of its "intransigent and punitive stance" Brexit can't be stopped.
"We believe you could also make clear that your preferred outcome is a free trade deal between Britain and the EU, an arrangement that is to the mutual benefit of both parties," the letter says.
It goes on to say that although a free deal commerce is "eminently possible", it believed it was "time" to transfer to a World Trade Deal below WTO guidelines "in light of the reluctance of the EU swiftly to secure a free trade deal".
Britain is due to go away on 29 March 2019, 46 years after it first joined the European Economic Community, the forerunner to the EU.
BBC political correspondent Ben Wright says the letter is "a sign of how frustrated hardline Brexiteers are becoming".
He provides: "The letter shows how intense the pressure is for Mrs May not to compromise, from some in her own party."
The UK needs to negotiate a "comprehensive, bold and ambitious" free commerce settlement with the EU.
If it might probably't obtain that, there are a variety of different potential preparations of various depth earlier than the UK reaches the purpose the place it has no preferential commerce relationship with the EU apart from widespread membership of the WTO.
If the UK had to commerce below WTO guidelines, tariffsa tax on traded itemscan be utilized to all UK exports.
The common WTO tariff varies from product to product, from zero% on mineral fuels and prescribed drugs, to round 20-35% on processed meals and 45-50% on meat.
Reality Check: Does the UK commerce with 'the remainder of world' on WTO guidelines?
The warning from well-known Brexit supporters comes after tens of 1000's of individuals marched in central London, demanding a ultimate vote on any UK exit deal..
On Friday, plane-maker Airbuswhich employs 14,000 folks in Britainmentioned it might go away the UK if it exits the one market and customs union with no transition deal.
Car maker BMW additionally warned that readability is required on a commerce deal by the top of the summer season, doubtlessly affecting the corporate's eight,000-strong employees within the UK.
In the letter, EFT says Britain can "flourish, even without a free trade deal, because of benefits of leaving the EU".
"This would give the chancellor ample scope to increase spending on priority public services such as the NHS, while reducing the too high UK tax burden."
Earlier this week, Theresa May mentioned a so-called Brexit dividend may very well be used to fund a part of an additional £20bn a yr for the well being service.
Although the Office for Budget Responsibility (OBR) calculates the UK will start saving £3bn a yr by 2020, this doesn't take note of Britain's anticipated £39bn divorce invoice for exiting the EU.
When requested concerning the EFT's letter, a Downing Street source mentioned: "We are confident of getting a good deal that delivers for every part of the UK and allows us to take back control of our money, rules and borders."