Theresa May called for Brexit talks to "accelerate and intensify" as she left an EU summit dominated by the migration crisis.
Although it appeared talks on how to tackle the issue had hit an impasse on Thursday, with Italy reportedly the cause of the deadlock, leaders finally reached an agreement in the early hours of Friday.
But while immigration was the focus of the meeting, the prime minister sought to keep Brexit on the agenda by firing a warning shot to her continental counterparts as she left.
Having warned the 27 other EU leaders during the summit about the potential risk to their citizens' safety if a Brexit deal on security was not reached, she told reporters the UK was ready to enter the next phase in the negotiations over its departure from the bloc.
"We are going to be publishing our white paper shortly and I want to see the negotiations accelerating and intensifying thereafter," she said, shortly after 5am.
The summit is to continue without Mrs May, who used the opportunity to raise the issue of Brexit over dinner before the row over migration flared on Thursday evening.
0:52If the UK was blocked from any involvement in key European law enforcement initiatives after Brexit she warned: "We would no longer be able to share real-time alerts for wanted persons, including serious criminals."
"Our collective ability to map terrorist networks across Europe and bring those responsible to justice would be reduced," the prime minister added.
A senior UK source said: "European negotiators have put obstacles in the way of reaching agreement on mapping the movement of terrorists and criminals and sharing vital information."
The prime minister will now seek to reach a consensus among her divided cabinet colleagues and so a final agreement can be reached at a crunch meeting being held in less than a fortnight at Chequers, her official country residence.
0:39Meanwhile, Irish prime minister Leo Varadkar told Sky News that Brussels, and Dublin, were fed up with waiting.
"We actually need to see the white paper from the British government as to what they believe the relationship should be like," he said before the Brussels meeting.
"And it is frustrating two years after the referendum that we still don't have a white paper from the UK government as to what they think the relationship should look like."
More from Brexit
While a pressing issue, Brexit was overshadowed by the migration crisis at the summit.
As Sky News' senior political correspondent Beth Rigby reports, the immigration row is a more immediate concern for German Chancellor Angela Merkel, who warned that the crisis could make or break the EU.
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