Cameron will on Thursday try to gain support for the special terms he wants for the U.K. from the European Union. Given the dominance of worries about Greece this week, and the migrant crisis in the region, he may find it difficult to get the discussion he wants.
The concessions he is asking for include limits on welfare payments for workers from non-U.K. European Union states and stronger guarantees that non-euro-zone members will not have rules forced on them by the single currency bloc. Cameron is also looking for greater impetus on trade deals outside the EU.
He has pledged to hold a referendum on EU membership by the end of 2017, but this could come earlier, potentially in 2016.
If the U.K. does leave the EU, the process could take up to a decade from now, as painstaking negotiations around its future relationship take place, according to new research from Global Counsel.
- By CNBC's Catherine Boyle