British Prime Minister David Cameron and his 27 EU counterparts struck a deal Friday on reforms aimed at keeping the country in the bloc, Lithuanian President Dalia Grybauskaite announced on Twitter.
The reform package is meant to convince Britons that they should vote to stay in the European Union, in a referendum widely expected to be held this year. Negotiations were protracted as leaders insisted that a deal could not come at any price.
"Agreement ... done. Drama over," Grybauskaite tweeted.
EU sources cautioned, however, that the deal was not entirely finalized.
A compromise proposal had been prepared for the leaders after a day of informal negotiations. An EU source, speaking on condition of anonymity, described the package of reforms as "very fair and balanced."
The leaders had been struggling to find a compromise between their refusal to budge on basic EU values and reforms on sovereignty, immigration, economic governance and competitiveness that Cameron said would allow him to campaign for Britain to remain in the bloc.
Tusk had spoken Thursday of a "make-or-break summit," with fears rife that the country is leaning towards Brexit - the buzzword coined for a British departure from the European Union after more than 40 years of half-hearted membership.
Many worry that this would destabilize the EU and reduce its global influence, at a time when the bloc is already struggling with a migration crisis and enduring economic woes.
One of the crunch issues had been a British demand to curb welfare payments to European workers, with tensions in particular over child benefits for those whose families live outside Britain.
Other tricky issues related to a British exemption to the EU's stated goal of "ever closer union," as well as a proposed mechanism for non-eurozone countries such as Britain to raise concerns about initiatives taken by the currency bloc.
Speculation has been rife that the deal will allow Cameron to call the in-out referendum for June 23.
The premier wrote earlier on Twitter that a British cabinet meeting will be held "if and when a deal is done."