A top Greek court has ruled that asylum-seekers newly arriving on Greek islands from Turkey should no longer be held there while their applications are assessed, court officials said on Wednesday.

Asylum-seekers have been prohibited from travelling beyond five islands since March 2016, when the European Union agreed a deal with Turkey to seal the sea route that nearly a million people, mostly Syrians, took in 2015.

Although the number of migrants and refugees crossing the narrow stretch between the Turkish coast and the islands remains well below 2015 levels, arrivals are up 27 percent this year compared with the same period in 2017, United Nations data shows.

The restriction on leaving the islands, imposed by Greece's Asylum Service, has resulted in severely overcrowded camps and violent protests over delays in asylum decisions. More than 15,000 asylum-seekers are living in five island camps, more than double their capacity, according to government data.

The Council of State, Greece's top administrative court, annulled the decision because it found no ‘serious and overriding reasons of public interest and migration policy to justify the imposition of restriction on movement’, a court official said.

The ruling, effective immediately, applies to new arrivals only and not to asylum-seekers who are already on the islands.

Greece's migration ministry said it would study the court's decision and that there was a provision ‘regulating the matter’ in a bill which has been sent to parliament.

Judges noted the islands had to manage a significant number of people seeking international protection while dealing with Greece's financial crisis. There were also risks of social tensions that could hurt the local economy because the islands are also tourist destinations.

The Greek Council for Refugees, which had taken the issue to court, said the decision was a ‘an important victory for all those who defend the rights of refugees in Greece and in Europe’.