However, even before the talks have begun, the European side has expressed concern that the British have underestimated just what is required to make such an agreement work.
The EU side has said that this willonly happen when "sufficient progress" has been made on phase one and that they will determine the level of progress.
Brexit Secretary David Davis will call for "a deal like no other in history" as he heads into talks with the EU.
With May still hammering out the details of a post-election deal to stay in power with the support of a small Northern Irish party, there are fears of a disorderly exit that would weaken the West, imperil Britain's $2.5 trillion economy and undermine London's position as the only financial centre to rival NY.
May is under pressure to show her leadership skills after the election and complaints about her response following last week's fatal tower fire in London.More news: Mamata called us terrorists, not ready for talks with WB govt: GJM
"What can you say of meaning about such chaos?" the diplomat asked.
While the two leaders held opposite views on the Brexit, they both agreed to a joint action plan that would see their two countries combine efforts to crack down on online terror propaganda and extremism.
11pm March 29 2019: The moment Britain ceases to be an European Union member.
Anxious by immigration and loss of sovereignty, Britain voted past year to end its four-decades-old membership of the 28-country bloc - the first state ever to do so - in a shock referendum result.
Currently, Europeans have the right to live, work, study and claim welfare benefits in Britain, as they do anywhere in the 28-nation union.
The Brexit secretary's comments come as formal negotiations between the United Kingdom and the European Union are set to commence, and shortly after European Union leaders such as French president Emmanuel Macron have attempted to encourage Remain campaign diehards by saying "the door is always open" to staying. Some of her ministers want to refashion her strategy toward protecting trade with Britain's biggest market rather than continue to aim for her original goal of winning control of immigration and law-making.
Opponents describe that as a "hard Brexit".More news: Hamilton Closes Gap In Formula One Championship Race
May has yet to change tack formally and doing so would risk infuriating eurosceptics in her Tory party who might still prove strong enough to scupper a deal or to topple her as leader if they believe she's backsliding.
"We have come together to urge the government to put the economy first as it prepares to start formal negotiations on the UK's departure from the European Union", the letter said.
London-based banking giant HSBC said Friday that it would keep more jobs in Britain depending on the government's approach to Brexit.
No. Except perhaps in agreeing a schedule for these talks and the separate strand of talks on "the future relationship" post-Brexit - ie trade.
But the true figure could be far lower, as the 100 billion does not account for tens of billions that Britain is set to get back in shared assets and rebates.
Britain seems to have tacitly accepted the EU's plan for sequenced talks, which will focus first on the terms of Britain's withdrawal, with negotiations on a future relationship and trade deal coming later.
But EU officials have warned her off trying to do this at an EU summit this Thursday, saying it is too soon.More news: Taliban attack kills five Afghan police, injures 18
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