Negotiations on a change to Britain's EU membership terms is continuing and a British cabinet meeting will not be possible on Friday, Prime Minister David Cameron has said on Twitter.
Cameron, locked in talks in Brussels with fellow EU leaders, said a cabinet meeting in London would be held "if and when a deal is done".
A cabinet meeting will be a crucial step for Cameron after any EU deal, as it will formalise the government's support for staying in the EU in an upcoming referendum.
Cameron is also expected to announce the date of the referendum, which must be held by the end of next year but has been widely expected to take place in June, after the cabinet meeting.
Cameron is expected to campaign for Britain to stay in, but he has announced that the cabinet's collective responsibility will be lifted on the EU issue, and a number of senior figures are expected to campaign for a so-called "Brexit".
The ruling Conservative Party has been divided over Britain's EU membership for decades.
He has struggled at a European Union summit to overcome resistance to a deal designed to keep Britain in the 28-nation bloc, with diplomats forecasting an agreement within hours.
"We are moving forward but we are not yet at a stage where a deal is almost done," a British official said after the UK prime minister held all-night negotiations in Brussels with top EU officials and a handful of leaders with specific objections to the draft text.
Cameron was hoping to fly home and chair a cabinet meeting later on Friday to endorse what he calls a "new settlement" with the EU, setting in motion plans to call a referendum on Britain's future in the union, probably for June 23.
The stakes are high for both Britain and the EU, with opinion polls showing voters almost evenly split.
All sides at the summit said the toughest issue remained Britain's drive to restrict welfare benefits for migrant workers from other EU countries, with east European states fighting to preserve the rights of expatriates already working in the UK and elsewhere.
Cameron has promised Britons he will exclude new European immigrants from in-work benefits for four years and cut child benefit for workers whose families remain at home.
French President Francois Hollande, backed by Germany, has pushed for amendments to ensure Britain cannot veto deeper integration by the euro zone countries or give City of London banks a competitive advantage through regulation.
Britain is the EU's second largest economy and is already the most semi-detached member, having opted out of joining the euro single currency, the Schengen zone of passport-free travel and many areas of police and judicial cooperation.
No country has ever voted to leave the union.
Britain's exit would end the vision of the EU as the natural home for European democracies and reverse the continent's post-World War Two march toward "ever closer union".