David Cameron has told his European Union counterparts he's prepared to fight a referendum on British membership of the bloc on the basis of unratified changes to its treaties.
The British Prime Minister, who yesterday started the formal process of renegotiation, will seek legally binding guarantees, his office said.
Opponents warned he runs the risk of the changes being rejected by parliaments and voters in other European countries after Britain has already decided whether to stay in the EU.
"I don't think jam tomorrow will be sufficient any more," Nigel Farage, leader of the UK Independence Party (Ukip), which campaigns for withdrawal from the EU, told reporters in Brussels.
"Why should we believe any future promises?"
Mr Cameron has pledged to win better terms and call a referendum by the end of 2017 on whether Britain should stay in the EU.
He wants to protect British sovereignty and ensure fair treatment for countries outside the euro area, while preserving the single market and imposing controls on migration within the bloc.
The reforms will need "full-on" change to the EU's governing treaties, Mr Cameron said on January 4, a process that can take many months for all 28 member states to ratify.