The European Commission will kick-off Brexit talks tomorrow as it meets to authorise the start of negotiations.
The Commission's general affairs council will rubber-stamp the negotiating guidelines agreed by the EU 27 nations at a meeting on 29 April, and nominate the Commission as the EU negotiator. However, talks will not start in earnest until after the UK General Election.
The negotiating directives will set the agenda for the first phase of Brexit negotiations, prioritising the rights of citizens and the Brexit bill.
Paul Hardy, Brexit director at DLA Piper, said: "The great game begins.
"We know the Commission will prioritise citizens' rights, the financial settlement and the border with Ireland. The clash with the UK starts here, given that the Commission refuses to commence talks on trade before these three issues are resolved.
"Given the UK government's desire to run a dual-track negotiation process (this may of course change post-8 June), Brexit may begin with a protracted battle on the terms of the negotiations themselves."
Brexit secretary David Davis has said that the UK will walk out on talks with the EU if it does not agree to set aside demands for a €100bn (£86bn) Brexit bill.
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Speaking to the Sunday Times, Davis said: "We don't need to just look like we can walk away, we need to be able to walk away.
"There are plenty of people in the European Union who want this to succeed. There may be some who want it to fail. You mustn't take everything as the unvarnished truth. I'm of the view that the likeliest outcome is the outcome we are looking for."
Several top European politicians, including German chancellor Angela Merkel, have said trade talks with the UK will only start once the bill is settled. However, economists have warned Merkel's government that a transitional trade arrangement with the UK should be agreed to prevent any economic shocks from Brexit.