David Davis will spearhead the UK's negotiating efforts.
Investors will also be waking up to news that one person has been killed and several injured after a man drove into pedestrians near Finsbury Park Mosque in the early hours of Monday morning.
Davis, however, told the press he was entering the negotiations with a "positive and constructive" frame of mind, and he will present a joint press conference this evening from Brussels with the EU's chief negotiator Michel Barnier.
German Chancellor Angela Merkel promised the European Union would listen carefully to Britain's wishes, saying it was in the interest of both sides to reach a good deal. The pound has fallen about 14 percent against the dollar since the June 23 referendum, pushing up inflation even as the economy shows signs of slowing.
There's little evidence Britons have changed their mind over leaving the EU.
"Both of us want to achieve the best possible outcome and the strongest possible partnership", Davis said. On Monday, he called for "a deal like no other in history".
"The question is not whether we're leaving the customs union, the question is what do we put in its place in order to deliver the objectives which the Prime Minister set out in the Lancaster House speech".
Transport and vehicles, textiles, and farming and agriculture would likely face higher tariffs under a "hard Brexit", according to Smith.
During the first day of talks, Mr Davis will say that the UK's desired "destination is clear - a deep and special partnership between the United Kingdom and the EU".
Labour's Shadow Brexit Secretary Sir Keir Starmer told the BBC's Pienaar's Politics said: "There's a lot of anxiety in the business community about stepping outside the customs union because it creates so many difficulties". Freedom of labor movement is a tenet of European Union membership and May initially interpreted the referendum result as a demand to stop that.
Davis's agreement to Monday's agenda led some European Union officials to believe that May's government may at last coming around to Brussels' view of how negotiations should be run. The EU wants to deal with the first phase of divorce talks before moving on next year to discuss trade, though EU officials acknowledge that the agreements to be reached before Britain leaves can only be concluded as a whole package simultaneously. Business leaders have already warned against such a "cliff edge" scenario and are eager to see a transitional period to give them time to adjust after the split. Such firms sold 7.3 billion euros ($8.2 billion) worth of goods to customers in Britain past year, their fourth-biggest market. Many young people also voted with an angry eye on Britain's exit, driven by thoughts of futures upended under a hard exit.
The UK may already have made one concession.
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".
Only when "sufficient progress" is made on these matters is Barnier willing to turn to the "bold and ambitious" trade deal that May wants, a pivot most doubt is unlikely to happen much before October.
Brexit Secretary David Davis arrived in Brussels to launch talks he hoped would produce a "new, deep and special partnership" with the EU.
Such is the collapse of May's authority that her entire Brexit strategy is being picked apart in public by her ministers, her lawmakers and her allies on the eve of formal negotiations which begin in Brussels today at 0900 GMT.
Olly Robbins: Senior Whitehall mandarin who will co-ordinate the officials involved in day-to-day negotiations.