EU countries united on Brexit negotiating guidelines

European Union leaders unanimously backed a tough Brexit strategy at a summit on Saturday, demanding a "serious response" from Britain on the rights of European citizens before trade talks can start.

In an unprecedented show of unity in Brussels, the 27 EU leaders unanimously and rapidly agreed on guidelines for the bloc's negotiations with Britain over its withdrawal from the bloc.

EU President Donald Tusk hailed the "outstanding unity" in an often fractious club, saying it was a "firm and fair political mandate" for the Brexit negotiations.

"After four minutes we were ready, it's very promising," former Polish premier Tusk told a news conference.

Tusk said the fate of three million EU citizens living in Britain and one million Britons on the continent "must be number-one priority for EU and the UK."

Britain swiftly warned that the talks could be "confrontational".

What is the next step?

Leaders say talks on a future trade deal with Britain can only start once London agrees divorce terms on citizens' rights, its exit bill, and Northern Ireland.

In a further move that will rile London, the EU is also set to back automatic membership for Northern Ireland if it reunifies with Ireland, and call for Spain to have a say over any deal that affects Gibraltar.

The leaders will also discuss for the first time the spoils of Brexit, in particular the relocation of EU medical and banking agencies that are currently based in London.

The EU guidelines say that only when "sufficient progress" has been made on divorce issues can trade talks begin, with sources saying they hope to do that by the end of the year.

EU leaders were discussing how to define this progress after approving the guidelines at the summit, EU officials said.

Actual Brexit talks are not expected to begin until after the British election, although the EU is set to give an official mandate to the EU's chief Brexit negotiator, Michel Barnier, on May 22.