However, the 27 insisted in their guidelines on the future trade deal that negotiations could only progress once the United Kingdom provides a legally watertight alternative to its "backstop" solution of keeping Northern Ireland in regulatory alignment with the EU. EU President Donald Tusk said the bloc would now use the "positive momentum" to tackle "outstanding issues", including how to avoid border checks between the British province of Northern Ireland and EU member Ireland. "We will now be sitting down and determining those workable solutions for Northern Ireland, but also for our future security partnership and economic partnership".
Speaking about the agreement on Brexit on Friday, UK Prime Minister Theresa May told reporters, "I welcome the fact that the EU Council this morning has agreed the details of the implementation period, that period after we leave the EU next March 2019 until the end of December 2020".
"This gives certainty to people and businesses".
"I believe we are approaching this with a spirit of cooperation, and spirit of opportunity for the future as well".
In a joint statement released by leaders after midnight Brussels time the European Union leaders said there was "no plausible alternative explanation" for the use of Novichok in Salisbury other than its deployment by the Russian government.
As well as future trade deals, the negotiating guidelines lay out how future security, defence and foreign policy ties will work.
The UK's "red lines" will limit the closeness of the future relationship.
Russian Federation on Friday slammed Britain for encouraging European Union member states to support London over a spy poisoning row after they agreed to recall the bloc's envoy from Moscow for consultations.
Mrs May claims that she can negotiate a Brexit deal that provides for trade that is "as frictionless as possible"; many in Brussels and some in Mrs May's own government believe that this can only be guaranteed if Britain remains in some form of customs union with the EU.
Speaking as he headed into the summit, Michel Barnier, Brussels' chief Brexit negotiator, said the European Union was "crossing a decisive point in this hard and extraordinary negotiation".
But she said what was needed was a permanent exemption, given the "huge importance" of the industry to the British economy.
The next phase of Brexit talks will focus heavily on Ireland and on Mrs May's promise to agree a "backstop" plan to ensure that there will be no reintroduction of a hard border between Northern Ireland and the Republic.