British Prime Minister David Cameron faces two months of difficult negotiations in his bid to alter European Union rules ahead of a planned referendum on Britain's continued membership in the EU.

Substantive differences remain, but Cameron and European leaders said after a grueling negotiating session that they would search for a compromise ahead of a February summit.

Cameron is calling for a change in rules that would limit the access of workers from other parts of the 28-nation bloc to benefits from the British welfare system.

European leaders say this would be discrimination contrary to the fundamental principle allowing the free movement of people within the EU.

Defense Secretary Michael Fallon told BBC Radio Friday Britain remains "deadly serious" about preventing its benefits system from attracting migrants.