Negotiations over a reform deal that is meant to keep Britain in the European Union were going into overtime Friday, with EU leaders not expected to be presented with a new proposal until the afternoon.
Hopes are high that a reform package will convince Britons that they should vote for their country to remain in the EU, in a referendum that is widely expected to take place this year.
But an initial round of summit discussions on Thursday evening had shown that still "a lot" remained to be done for all of the bloc's 28 leaders to endorse the package, EU President Donald Tusk said.
"Work ongoing on revised UK in EU settlement," Tusk's spokesman, Preben Aaman, wrote on Twitter on Friday morning, saying that leaders would hold small-group talks from 11 am (1000 GMT) before reconvening for an "English lunch" at 1:30 pm.
The leaders had initially planned on meeting again for breakfast.
Tusk has spoken of a "make or break summit," with fears rife that Britain is heading towards Brexit - the buzzword coined for a British departure from the EU after more than 40 years of half-hearted membership.
Many worry that this would destabilize the EU and reduce the bloc's global influence, at a time when it is already struggling with a migration crisis and enduring economic woes.
British Prime Minister David Cameron has demanded EU reforms in the areas of sovereignty, immigration, economic governance and competitiveness, which he has said would allow him to campaign for Britain to stay in the bloc.
But the 27 other member states have refused to budge on basic values, particularly the freedom to live and work anywhere in the EU.
One of the crunch issues has 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.
London is keen to limit the number of EU workers seeking jobs in Britain, amid a perception that continental Europeans are saturating its labour market.
Just over 2 million EU citizens were working in Britain at the end of 2015, out of a total of around 31 million workers, according to national statistics released Wednesday. The country has the fifth-largest economy in the world.
Other issues unresolved early Friday had 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 if a deal is reached at the Brussels summit on Friday, it will pave the way for Cameron to call the in-out referendum for June 23.