European Council chief Donald Tusk says talks Sunday in London with British Prime Minister David Cameron broke up without a deal to keep Britain in the EU, but he said "intensive" negotiations will continue Monday.
Tusk, tweeting late Sunday, described talks in the next 24 hours as "crucial." Cameron said Monday's meeting likely will lead to a draft text of EU reforms demanded by Britain's ruling conservatives in exchange for remaining in the 28-nation political and economic bloc.
Officials on both sides of the issue have voiced hopes of striking a deal ahead of a two-day EU summit set to begin February 18 in Brussels.
If a deal is reached, it then would need approval of British voters in a referendum that Cameron's party has promised by the end of 2017.
Discord between London and Brussels grew last year as Europe's migrant crisis gathered strength and swept through the continent. Cameron's government has since demanded EU reforms aimed in part at allowing EU countries to control the number of migrants entering EU states and to limit their welfare benefits.
On Friday, EU officials offered London a mechanism known as an "emergency brake" that would let Britain temporarily limit such benefits, if the country's welfare coffers are strained.
But Cameron later described the offer as "not good enough." He is said to be seeking further concessions that would allow the braking mechanism to take effect sooner and remain in place longer than the EU has proposed.
London also is said to be demanding more EU power ceded to the union's national governments, and protections for Britain and eight other EU countries that do not use the euro currency.