European Union leaders unanimously agreed on negotiating guidelines for Brexit talks with the UK on Saturday at a special summit in Brussels, European Council President Donald Tusk confirmed via Twitter.

This is the first summit of 27 EU countries leaders without Britain since British Prime Minister Theresa May triggered the Article 50 last month.

Leaders approved the draft guidelines within one minute during the summit, which were issued by Tusk on March 31.

In addition, they also promised to embrace Northern Ireland into the EU if a referendum sees it leave the United Kingdom and unite with the Irish Republic. During the summit, Irish Taoiseach Enda Kenny asked fellow members to acknowledge that Northern Ireland would, like East Germany in 1990, automatically enter the EU in the event of unification with the existing member state.

A 2016 BBC survey held after the Brexit referendum found that only 22 percent of people in Northern Ireland would support joining the Republic of Ireland. Under the Good Friday Agreement, a border poll can be called if a clear majority of people in Northern Ireland calls for a united Ireland.

Talks with Downing Street will begin after the UK's general election on June 8.

The 27 leaders agreed that 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.

The deadline for completing the negotiations is March 29, 2019.

Tusk earlier said leaders "need to remain united as the EU 27," but said it was "also in Britain's interest" if unity boosted the chances of a swift Brexit deal.‍

French President Francois Hollande said as he arrived that "the aim of the summit is unity", adding that "there will inevitably be a price and a cost for Britain."

German Chancellor Angela Merkel said the EU wants "good relations" with Britain, but added that "we also want to defend, at 27, our common interests - so far we have done extremely well."

The EU's chief Brexit negotiator, Michel Barnier, said the bloc had a "clear line" on the talks. "But this unity is not directed against Britain, I think that it is also in its interest," he said.

The EU 27 have considerably toughened the guidelines since Tusk first unveiled them a month ago, with Brussels also drawing up a detailed list of citizens' rights.

Tusk said this issue - 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."

Officials hope for agreement on this point by the end of the year.

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 calls 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 - 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 these 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.