EU leaders have approved guidelines for the negotiation of future relations with the UK after Brexit.
The UK is due to leave in March 2019 and negotiators have said they want a deal in place by the end of the year.
Prime Minister Theresa May has said she wants a "new dynamic" in talks on an economic and security partnership.
The formal adoption of the guidelines, although widely expected, is seen as another key step as the Brexit process gathers momentum.
The guidelines give chief negotiator Michel Barnier the mandate to talk directly to the UK about the future relationship with a view to reaching a broad political agreement by October to allow the EU and UK parliaments time to consider it.
Announcing the move on social media, European Council President Donald Tusk tweeted:
Earlier, UK PM Theresa May told reporters:"We've made good progress on the withdrawal agreement.
"But also I'm looking for a new dynamic in the next stage of the negotiations so that we can ensure that we do develop, that we work together to develop, a strong future economic and security partnership which I believe is in the interest of the UK and the European Union."
At a dinner on Thursday night, Mrs May said the UK and EU should work on issues like Northern Ireland, trade and security with "energy and ambition".
"We have the chance now to work together to explore workable solutions - in Northern Ireland, in our future security co-operation and in order to ensure the future prosperity of all our people."
She added:"This is an opportunity.It is our duty to take it and to enter into it with energy and ambition."
Later on Friday, the remaining 27 leaders are set to endorse a 21-month transition period after March 2019, when the UK officially leaves.
The UK is set to leave the bloc on 29 March 2019, but earlier this week Brexit Secretary David Davis and EU chief negotiator Michel Barnier struck a deal that would allow for a transition period - which the UK government describes as an implementation period - until December 2020.
Under the terms of that joint legal text, the UK will be able to negotiate, sign and ratify its own trade deals, while EU citizens arriving in the UK will enjoy the same rights and guarantees as those who arrive before Brexit.
A solution to avoid a hard border on the island of Ireland has yet to be agreed, with the EU insisting on a "backstop" option of Northern Ireland effectively remaining in the customs union.
EU Council president Donald Tusk said on Thursday he was "absolutely sure" a solution would be found to prevent the return of physical checks on the border between Northern Ireland and the Republic of Ireland.
Mrs May has remained in Brussels for the second day of the EU Council summit to discuss steel and other trade issues although she will not be present when her colleagues meet to rubber stamp the transition period and consider approving guidelines for the second phase of negotiations.
She welcomed the temporary US exemption for EU firms from tariffs on steel imports but said what was needed was a permanent exemption, given the "huge importance" of the industry to the British economy.
"I want to ensure that steel workers and their jobs are properly safeguard," she said.
The Trump administration has decided to temporarily exclude the EU, as well as a number of other countries, from paying tariffs of 25% on steel imports due to come into effect on Friday....