As EU leaders prepared to endorse a deal on the UK's post-Brexit transition, Prime Minister Theresa May called for a "new dynamic" in negotiations on Britain's future relations with its continental partners.
Leaders of the 27 remaining member states were expected on Friday to clear the way for talks on future trade and security to begin, by endorsing the transition deal struck by Brexit Secretary David Davis and chief EU negotiator Michel Barnier earlier this week.
Addressing the EU27 over dinner in Brussels on Thursday evening, Mrs May said it was their duty to show "energy and ambition" in forging a future relationship which was in the interests of both Britain and Europe.
The Prime Minister extended her visit to the European Council summit in Brussels in order to take part in discussions on US trade tariffs on Friday morning, but she will leave the meeting to allow the EU27 to make their momentous decision on Brexit in her absence.
Arriving at the summit venue, Mrs May said "good progress" had been made on negotiating the terms of the UK's withdrawal from the EU.
And she added: "I am looking for a new dynamic in the next stage of negotiations, so that we can ensure that we work together to negotiate and develop that strong future economic and security partnership which I believe is in the interests of the UK and the EU."
Mr Barnier, who kissed Mrs May's hand as they met in the entrance lobby of the Europa building in Brussels, said Friday marked a "decisive" moment in "this difficult and extraordinary negotiation".
He cautioned that the future partnership negotiated over the coming months "must respect the principles and identity of the EU and the single market".
European Council president Donald Tusk raised doubts earlier this week over whether a deal reached on Monday would be acceptable to all 27 remaining members - with Spain reported to be holding out to improve its hand over Gibraltar.
But a senior UK official said on Thursday evening that "all the indications are positive" that the transition deal will get the thumbs-up.
Addressing fellow leaders over dinner, Mrs May said "compromise on both sides" had been required to reach a mutually acceptable legal text on the transition, which will see the UK continue to observe EU rules until the end of 2020 while winning the freedom to negotiate and ratify trade deals with outside countries.
Britain believes that moving on to trade talks with the EU will help unblock the logjam over the Irish border and prevent the need to implement a "backstop" solution proposed by Brussels which would keep Northern Ireland in the customs union.
London is hopeful that a solution to keep Northern Ireland's border with the Republic open will emerge in the context of a deal allowing trade between Britain and the rest of the EU to be as frictionless as possible.
Mrs May told the EU27 leaders: "We have the chance now to create a new dynamic in the talks, to work together to explore workable solutions on Northern Ireland, on our future security co-operation and in order to ensure the future prosperity of all our people.
"This is an opportunity it is our duty to take and to enter into with energy and ambition."
Irish Taioseach Leo Varadkar said he envisaged a trading relationship between the UK and the EU "so close that many of the measures in the backstop may become unnecessary".
Mr Tusk has recommended EU27 endorsement of the transition deal, which he said would put off "the negative consequences of Brexit" for another 21 months after the formal date for the UK's withdrawal in March 2019.