And Britain's divorce deal and payout to the European Union must be agreed before trade talks begin.
The United Kingdom's negotiators would not have chosen to start two years of Brexit talks amid Conservative threats of war with Spain over Gibraltar.
"People can not end up being simply a negotiating chip", said European Commission Chief Jean-Claude Juncker.
That is apparently what European Council President Donald Tusk and British Prime Minister Theresa May have agreed to do when it comes to the upcoming Brexit talks.
An EU official said Tusk and May had a "good and friendly" meeting lasting almost two hours at the British leader's Downing Street official residence.
However, Downing Street later said the Prime Minister had made clear to Mr Tusk that there could be no change to the status of the British enclave without the consent of its people.
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"May and Tusk agreed during their phone call on 28 March to establish a regular contact at the highest political level to create a channel for facilitating the Brexit process outside the day-to-day talks", the source said.
The European Parliament became the first EU institution to set out its guidelines for Brexit talks on Wednesday, in which it said guaranteeing the rights EU and British citizens living overseas should be a priority. Britain wants the two strands to go hand-in-hand.
But they omitted any mention of the flashpoint issue of Gibraltar, unlike Mr Tusk's guidelines, which said Spain should have the final say over whether any eventual trade deal applies to the British outcrop.
It was the first time the two had met since May triggered the Article 50 process last week, setting in motion Britain's departure from the European Union (EU) through an official letter delivered to Tusk on Britain's decision to leave the bloc.
Full negotiations are expected to start in late May once the negotiating guidelines of the EU's 27 remaining nations have been sealed in a mandate for the EU's chief negotiator, Michel Barnier. At the end of this negotiation, will we have looked at both withdrawal and the future relationship?
Mr Tusk has indicated that post-Brexit trade deal talks could begin this autumn, but only if "sufficient progress" has been achieved in negotiations on the arrangements for Britain's withdrawal - widely seen as a reference to agreement on a "divorce bill" of an estimated £50 billion set to be presented to the UK.
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