Prime Minister David Cameron is bracing for a crucial summit Thursday with continued British membership in the European Union on the line, convinced he needs to clinch a deal to fundamentally reform the bloc to make sure a membership referendum can be won.

With as many as four core issues still outstanding going into the two-day summit of EU leaders, EU President Donald Tusk said on the eve of the start that "frankly: there is still no guarantee that we will reach an agreement."

The lingering disputes over anything from the reinforcement of national sovereignty to the indexation of child benefits belie the fact that none of other member states wants to see the departure of Britain, an economic and diplomatic giant in a struggling EU.