Brexit Secretary David Davis will call for “a deal like no other in history” as he heads into talks with the EU.
Subjects for the negotiations, which officially start in Brussels, include the status of expats, the UK’s “divorce bill” and the Northern Ireland border.
Mr Davis said there was a “long road ahead” but predicted a “deep and special partnership”.
The UK is set to leave the EU by the end of March 2019. The talks are due to get under way at 10:00 BST.
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Day one of the negotiations, at the European Commission buildings in Brussels, will be followed by a joint press conference this evening by Mr Davis and the EU’s chief negotiator Michel Barnier, a former French foreign minister and EU commissioner.
The BBC’s Europe editor Katya Adler said the opening session would focus on basic issues of procedure such as how often the two men and their teams will meet and in what order items will be discussed.
Above all, she added, it would be a “trust-building exercise” after all the “mud-throwing” of recent months.
The UK minister, who will be accompanied by a team of British officials, is expected to say: “Today marks the start of negotiations that will shape the future of the European Union and the United Kingdom, and the lives of our citizens.
“We want both sides to emerge strong and prosperous, capable of projecting our shared European values, leading in the world, and demonstrating our resolve to protect the security of our citizens.
“I want to reiterate at the outset of these talks that the UK will remain a committed partner and ally of our friends across the continent.
“And while there is a long road ahead, our destination is clear – a deep and special partnership between the UK and the EU. A deal like no other in history.”
The BBC has been told by European Union sources that the talks will follow the EU’s preferred pattern of exit negotiations first, with the future relations between the two sides – including the free trade deal the UK is seeking – at a later date.
Five major UK business bodies have come together to call for continued access to the European single market until a final Brexit deal is made with the EU.
In a letter to Business Secretary Greg Clark they urged the government to “put the economy first”.
The letter is from the British Chambers of Commerce, Confederation of British Industry, EEF, Federation of Small Businesses and Institute of Directors.
On the eve of talks, Chancellor Philip Hammond issued a warning about the implications of the UK leaving the EU without a deal in place, telling the BBC’s Andrew Marr Show that having no deal would be “a very, very bad outcome for Britain”.
Environment Secretary Michael Gove said he believed a “good deal” could be struck but that Theresa May was right to insist that the UK would not sign up to anything which “harmed” the UK.
He told BBC Radio 4’s Today that by leaving the EU, the UK could “revive” its fishing industry and provide “better protection” for farmers. Asked whether his claim during last year’s referendum that Brexit would reduce food prices was deliverable, he replied “yes, I do”.
He suggested there was “no tension” between economic and social imperatives when it came to immigration, saying Brexit would give the UK “democratic control of our borders” and the freedom to decide numbers “in accordance with economic and other needs”.
Former Marks and Spencer chairman Lord Rose, who chaired the Stronger In campaign last year, said he was reassured that economic considerations were “top of the pile” but ministers needed to be realistic with the public.
“Let’s communicate with people who voted Out and people who voted Remain what the art of the possible… we all know we can’t have our cake and eat it… negotiations mean you are not always going to get what you want.”
Speaking on the same programme, JD Wetherspoon founder Tim Martin – one of the leading pro-Leave business voices – said negotiators had to be open to possible compromises but also prepared to walk away and to default to World Trade Organisation rules if necessary.
“I don’t think many people feel that staying in the single market and customs union and being subject to EU laws is Brexit. I think Brexit is parliamentary sovereignty and an assertion of democracy. Outside that, I think there is a quite a lot of scope.”
Brexit negotiations: David Davis targets ‘historic’ deal}