Brexit Secretary David Davis will call for a "deal like no other in history" as he heads to Brussels to launch negotiations for Britain's exit from the European Union.
Talks with the European Commission's chief negotiator Michel Barnier will focus on the status of expats, the UK's "divorce bill" and the Northern Ireland border.
But officials insisted the UK would continue to push for an agreement on trade relations to be dealt with alongside a deal on the withdrawal process.
It comes as Philip Hammond warned failing to secure a Brexit deal would be "very, very bad" for the country and insisted there must be transitional arrangements to avoid a "cliff edge".
Mr Davis will be accompanied by a nine-strong negotiating team that includes the most senior civil servants at the department as well as officials from the Treasury and Home Office as well as Mark Sedwill, the national security adviser to the Prime Minister.
The Brexit Secretary 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.
"I look forward to beginning work on that new future today."
The agenda for the meeting was agreed earlier this month following preparatory "talks about talks".
The European Commission released a statement at the time saying discussions would focus on "issues related to citizens' rights, the financial settlement, the Northern Irish border and other separation issues, as part of the sequenced approach to the talks".
But the government still wants to negotiate its future trade relationship with the EU "alongside" talks on the terms for Brexit.
A spokesman for the Department for Exiting the European said: "We believe that the withdrawal process cannot be concluded without the future relationship also being taken into account."
Mr Hammond indicated temporary exit measures could be put in place for a couple of years before a final deal is sealed.
The Chancellor told BBC One's Andrew Marr Show: "What we put in place may not be a single arrangement that endures forever, it may be an arrangement which lasts for a couple of years as a temporary measure before we get to the long-term agreed status quo.
"We're leaving the EU and because we are leaving the EU, we will be leaving the single market and by the way, we will be leaving the customs union.
The question is not whether we are leaving the customs union. The question is what do we put in its place in order to deliver the objectives the Prime Minister set out in her Lancaster House speech of having no hard land border in Ireland and enabling British goods to flow freely backwards and forwards across the border with the European Union?
"It's a statement of common sense that if we are going to radically change the way we work together, we need to get there via a slope, not a cliff edge."
Mr Hammond said he would not agree to a deal that would "destroy" Britain.
"No deal would be a very, very bad outcome for Britain, but there is a possible worse outcome and that is a deal that is deliberately structured to suck the lifeblood out of our economy over a period of time."
Prime Minister Theresa May has repeatedly insisted the Government is prepared to walk away from the talks, claiming no deal is better than a bad deal.
Mr Hammond's warnings over Brexit were seized on by critics, with shadow chancellor John McDonnell claiming the Government is in "disarray".