David Cameron has struggled to overcome resistance to a deal designed to keep Britain in the EU.
"We are not yet at a stage where a deal is almost done," a British official said after all-night negotiations in Brussels with EU officials and a handful of leaders.
Cameron was hoping to fly home and chair a cabinet meeting to endorse 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.
Opinion polls show voters are 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. East European states are fighting for the rights of expatriates in the UK.
