Fast Facts: What to know about Britain's divorce of the EU

However, an agreement on the future relationship can only be concluded once the United Kingdom has formally exited the EU.

One particularly tricky aspect for May will be demand to spell out the UK's relationship with the European Court of Justice early in the talks, and how EU law applied to the exit deal, during any interim arrangement, and in the longer term.

He said that four core issues need to be resolved before moving on to talks about the post-Brexit arrangement: citizens's rights; avoiding "a legal vacuum for companies that stems from the fact that after Brexit the European Union laws will no longer apply to the UK"; a financial settlement; and the Irish border issue.

On Thursday, Mr Tusk issued a thinly veiled warning to the United Kingdom not to try and play divide-and-rule with the other 27 member states as talks begin.

France's foreign minister said talks for a future relationship can only take place once Brexit negotiations have concluded.

But no-one doubts the difficulties ahead, with Maltese Prime Minister adding that "it will be a tough negotiation, but it will not be a war".

Tusk's guidelines will now be sent out to the leaders of the 27 remaining European Union countries, which will suggest changes ahead of a special summit in Brussels to approve the plans on April 29.

Mr Tusk's guidelines appeared to offer a small compromise, saying that some trade talks could eventually be held at the same time, but he insisted on a "phased approach" during the Brexit process.

In the draft guidelines sent to member states, the European Union says that its "overall objective in these negotiations will be to preserve its interests, those of its member states, its citizens and its businesses".

Davis says: "We want a deal, and she was making the point that it's bad for both of us if we don't have a deal".

Members of the European Parliament want citizens rights to be discussed at the beginning, fearing their rights could otherwise be used as bargaining chips during the negotiations.

Tusk speaking in Valletta, Malta on Friday morning.

In her letter to Tusk formally giving notice to leave on Wednesday, May on several occasions said the talks on a new trade deal could happen while the exit negotiations happened.

In the first signs of a business fallout since May's Brexit notification, the prestigious Lloyd's of London insurance market said it would open a new Brussels subsidiary to ensure smooth operations in the EU.

Unveiling the government's white paper on the "Great Repeal Bill", Brexit secretary David Davis told the House of Commons that as well as transposing aspects of European Union legislation into United Kingdom law, the bill would create a new power to "correct the statute book". Britons voted by 52-48 percent in favour of Brexit in the June 2016 referendum.