Talks between the European Union and Turkey have laid the ground for a possible breakthrough deal aimed at stemming the flow of migrants into the continent, David Cameron has said.
Negotiations in Brussels broke up after more than 12 hours of discussions without a final agreement.
But Mr Cameron said a proposal to return all migrants who make it across the Aegean to Greece could provide the basis of a settlement that would finally close the refugee trail through the Balkans.
"It has been a long and difficult evening but I think we do have the basis for a breakthrough which is the possibility that in future, all migrants who arrive in Greece will be returned to Turkey," he said.
"That would, if implemented, break the business model of the people smugglers and end the link between getting in a boat and getting settlement in Europe.
"That is something that I have been arguing for for a year and I think this is significant but only if it is fully implemented and that's what needs to happen next."