A ruling by the EU's top court has dealt a further blow to Italy's attempts to create a fast-track system in Albania for processing asylum applications offshore.

Europe's top court on Friday questioned the legitimacy of the "safe countries" list Italy uses to send migrants to Albania and fast-track their asylum claims, in a fresh blow to a key plank of the government's migration policy.

The ruling follows preliminary questions raised by judges at the Rome Tribunal regarding the legality of detaining migrants rescued in the Mediterranean and then transferred to Albania