(Adds Truss comments)
By Elizabeth Piper and Andrew MacAskill
LONDON, March 2 (Reuters) - Britain will unveil on Monday its mandate for trade talks with the United States, with Prime Minister Boris Johnson vowing to drive a hard bargain in negotiations that are set to test the two countries' "special relationship".
After leaving the European Union in January, Johnson wants to pursue a trade deal with the U.S. to try to not only champion Britain's new independence but also to put pressure on the bloc in separate talks on a future relationship.
As a negotiating team heads to Brussels to start those talks, trade minister Liz Truss will set out the terms of the government's mandate with the United States, warning Washington that London would walk away if its demands are not met.
That could set the two sides on a collision course over food standards and access to Britain's much-loved National Health Service, putting additional pressure on a relationship already strained by London's decision to allow China's Huawei a limited role in its mobile network and a proposed digital services tax.
"We have the best negotiators in the business and of course, we're going to drive a hard bargain to boost British industry," Johnson said. "Most importantly, this transatlantic trade deal will reflect the unique closeness of our two great nations."
The government said its analysis showed a deal with the United States would boost transatlantic trade by 15.3 billion pounds ($19.61 billion), and add 3.4 billion pounds to the British economy. Britain's overall economy is worth around $2.7 trillion.
The United States is currently Britain's biggest trading partner after the EU, accounting for nearly 19% of all its exports in 2018 and 11% of imports. By comparison, the EU accounted for 45% of all UK exports and 53% of UK imports.
SCOTTISH SALMON FOR STETSON HATS
The government said manufacturers of cars, ceramics, food and drink, and professional services including architects and lawyers, would be among the biggest winners from the trade deal.
"Trading Scottish smoked salmon for Stetson hats, we will deliver lower prices and more choice for our shoppers," Johnson said.
Both sides hope a deal can be reached as soon as this year, but there are many hurdles.
The government reiterated that its National Health Service (NHS) was "not for sale" - addressing criticism that a deal could let private U.S. healthcare providers into Britain's state-funded health system.
It also vowed to uphold high standards on food safety and animal welfare amid fears from farmers that the government is ready to allow U.S.-produced chlorine-washed chicken and hormone-treated beef into Britain.
"In a trade deal with the U.S., we will not diminish our food safety standards and we will also not put the NHS on the table," Truss said. "If we don't get the deal we want we will be prepared to walk away."
The U.S. negotiating objectives published last year include pressing for full market access for U.S. pharmaceutical products and medical devices, which would require changes to the NHS pricing restrictions and could increase the cost of drugs.
Britain's Chief Trade Negotiation Adviser, Crawford Falconer, who previously worked as New Zealand's chief negotiator, will represent the government in the talks.
Britain also plans to begin negotiating deals with the United States, Japan, Australia and New Zealand in the coming months, alongside talks with the EU on future relations.
The government said its aim is to have 80% of external trade covered by free trade agreements by 2022. ($1 = 0.7800 pounds) (Reporting by Elizabeth Piper and Andrew MacAskill Editing by William James, Mark Heinrich and Alex Richardson)
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