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 later, 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".
Day one of the negotiations will start at about 11:00 BST at European Commission buildings in Brussels.
Mr Davis and the EU's chief negotiator Michel Barnier, a former French foreign minister and EU commissioner, will give a joint press conference at the end of the day.
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.
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 strong warning about the implications of the UK leaving the EU without a deal in place.
Mr Hammond told the BBC's Andrew Marr Show that having no deal would be "a very, very bad outcome for Britain" but added that one that aimed to "suck the lifeblood out of our economy over a period of time" would be even worse.
He called for a transition deal to be in place to avoid businesses being affected by a "cliff edge" scenario as the UK leaves.
Mr Hammond has said the UK should "prioritise protecting jobs, protecting economic growth and protecting prosperity".