Migrant-rescue ships win greater EU freedoms

And so the number of rescued people on board "must not be taken into account when verifying whether the rules on safety at sea have been complied with," it added.

Port authorities do have the right to carry out safety inspections on rescue vessels, but only if local courts first say "there are serious indications of a danger to health, safety, on-board working conditions or the environment".

And local courts, when taking such a decision, can "take account" of the fact "ships classified and certified as cargo ships by the flag state are, in practice, being systematically used for activities relating to the search for and rescue of persons".

If inspections find "deficiencies" port authorities can also adopt "corrective measures", such as detention of ships, so long as those measures are "suitable, necessary, and proportionate", the EU court added.

The case arose after German charity Sea Watch sued the port authorities of Palermo and Empedocle in Italy at a regional court in Sicily last year.

Two of the charity's vessels — Sea Watch 3 and Sea Watch 4 — are registered as cargo ships under German flags.

And Italian harbour masters had insisted on keeping them at port on grounds they had taken more people on board than their cargo certificates allowed.

Many NGO ships operate in the Central Mediterranean near Libya.

Some 25,164 people used this route to try to reach Europe in the first six months of this year — a 23 percent increase on the same period last year, according to Frontex, the EU's border control agency.

More than 30,000 other people so far this year also underwent perilous sea crossings in the West African, Western Mediterranean, and Eastern Mediterranean regions.

Most of those coming are from Algeria, Bangladesh, Congo, Egypt, Nigeria, Morocco, Syria, and Tunisia.

At least 978 migrants died trying to reach Spain alone this year — amounting to five people a day — according to Spanish NGO Caminando Fronteras.

Another 600 people died in the first three months of this year trying to reach Europe from Tunisia and Libya, the International Organization for Migration said.

"UNHCR [a UN humanitarian agency] has continuously been warning of the horrific experiences and dangers faced by refugees and migrants who resort to these journeys," UNHCR spokeswoman Shabia Mantoo said in June.

"Each year, thousands perish or go missing at sea without a trace," she said.