BRUSSELS (Reuters) - British Prime Minister Theresa May urged EU leaders to seize on “a new dynamic” in Brexit talks and solve differences over a future border with Ireland to agree a deal on their future partnership.

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Leaders of the European Union are expected to sign off on Friday on an offer of a transition period and set out their terms for forthcoming talks on post-Brexit trade. At the end of a Thursday night summit dinner, May took to the floor to call on them not to squander that opportunity to reach an overall deal.

By securing agreement on a 21-month transition period to allow businesses to prepare after Britain leaves the bloc in March next year, May now faces possibly the hardest part of a negotiation that has already shaken her grip on power.

“We have the chance, now, to create a new dynamic in the talks to work together to explore workable solutions -- in Northern Ireland, in our future security cooperation and in order to ensure the future prosperity of all our people,” she told the leaders at the end of a dinner of scallops and lamb.

“This is an opportunity it is our duty to take and to enter into with energy and ambition,” she added, according to a senior British official.

May hopes the expected agreement by EU leaders on Friday will mark the start of the end of punishing talks, during which she has been criticized by some in her own party for giving too much away to Brussels and failing to take a tough enough stance.

Some critics said the tone had been set for those talks when the lights went out at a meeting she had called with German Chancellor Angela Merkel and French President Emmanuel Macron to discuss an attack on a former Moscow spy in the English city of Salisbury which Britain has blamed on Russia.

BORDER

May, under pressure from Brexit supporters in her Conservative Party for settling for a standstill transition with the EU when almost nothing changes, has long wanted to discuss future ties with the bloc, a conversation her government believes can settle some of the thorniest issues.

One such problem is the border between Northern Ireland and EU member Ireland, which has been all but invisible since a peace agreement was signed to end decades of sectarian conflict in the North.

Both London and Brussels agree there should be no hard border, but they differ over how that can be achieved.

May has suggested a customs partnership or a streamlined arrangement, where jointly implemented measures and the use of technology minimize friction. EU officials reply by saying the easiest way to secure that would be to stay in its customs union -- something the British leader has ruled out.

The standoff has left a backstop agreement -- that could see the British province of Northern Ireland staying in the EU’s customs union -- still in the transition text, although subject to negotiators finding an ‘operational’ compromise.

That, for some Brexit campaigners, is a major concern.

But a senior British official voiced confidence the difficulties over the border could be resolved now that the two sides were talking about the future.

“Once we move on to this stage of being able to talk about the future relationship, that is where we believe the solution to the Irish border will be found,” he told reporters. “It will be a solution in terms of our entire relationship with the EU.”