EU leaders have told the UK they are willing to hold trade talks before Brexit but only if there is "sufficient progress" on the divorce negotiations.
The UK must settle its Brexit bill and reach agreement on the rights of EU citizens living in Britain before negotiations can move forward to trade talks, under guidelines set out in Brussels' approach to the upcoming discussions.
The European Council offered what EU sources described as a "carrot" with a two-phase approach: progress on divorce terms first, then trade talks.
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Setting out the guidelines , the Council's President Donald Tusk said: "Once and only we have achieved sufficient progress on the withdrawal can we discuss the framework for our future relationship.
"Starting parallel talks on all issues at the same time, as suggested by some in the UK, will not happen."
Mr Tusk set out four areas that must be agreed in the withdrawal phase.
These included the rights of EU citizens living in Britain and UK citizens in Europe, the Brexit divorce bill, which has been put at €60bn (£52bn), the border between Northern Ireland and the Republic and clarity over EU law in the UK after it leaves.
Mr Tusk said it was "only fair" the UK should honour the financial promises it made as part of the 28-member bloc.
However, speaking to Sky News, the Maltese president, Joseph Muscat, who holds the rotating presidency of the Council of the European Union said the final bill need not be calculated before trade talks but there must be agreement on how it is worked out.
The guidelines also make clear the EU wants to see a "transitional" deal after Brexit in 2019 and before any trade deal is agreed, but on the condition the UK continues to accept the bloc's rules.
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These will include paying budget contributions, remaining under the jurisdiction of the European Court and perhaps even accepting freedom of movement - significant issues for those who voted leave .
Mr Tusk also said there would be "no punitive approach, Brexit is punitive enough".
But he warned Theresa May she should not try to pick off countries separately to negotiate in a divide-and-rule approach and should negotiate "only with the 27 as a unity".
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Negotiations are expected to start on May 22.
Speaking as he arrived for a NATO summit in Brussels, Foreign Secretary Boris Johnson said: "We really are moving forward now, and there's a lot of goodwill, a lot of willingness to achieve what the Prime Minister has said she wants to
achieve, which is an orderly transition and then a deep and special partnership between a strong EU and a strong UK."
Liberal Democrat leader Tim Farron said the guidelines showed "the strength of the EU in these negotiations and the carelessness of the UK Government in isolating themselves from our European allies".
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On Wednesday, the Prime Minister set out her priorities for Brexit talks in a letter which triggered Article 50, in which she said divorce talks and free trade talks should take place at the same time.
However, this was immediately rejected by both German chancellor Angela Merkel and the French president Francois Hollande.
A Government spokesman said: "It is clear both sides wish to approach these talks constructively, and as the Prime Minister said this week, wish to ensure a deep and special partnership between the UK and the European Union."