The Italian coastguard has stopped a migrant rescue boat belonging to the German group Sea-Eye from operating due to safety breaches, it said on Saturday.
"An inspection revealed various technical irregularities which could compromise not only the safety of crew members but also those who have been rescued," the coastguard said in a statement.
Among other things "the inspection confirmed that the ship's life-saving equipment is sufficient for a maximum number of 27 people... which means that in an emergency the crew would not be able" to guarantee the evacuation of all the people on board, it added.
It said the ship was, as of Friday, the subject of an "administrative immobilisation until the rectification of the noted irregularities".
The German group's hospital ship was anchored in the port of Palermo after disembarking 415 migrants in the Sicilian port of Pozzallo and observing a period of quarantine imposed by the health authorities.
In a statement to AFP on Saturday, the German non-governmental organisation condemned the coastguard's decision.
"As for other civilian rescue ships, the same technical reasons are now invoked to put an end to the Sea-Eye 4's missions ," it said.
"The argument is still that the German rescue ships routinely save too many people from drowning," said the NGO's president Gorden Isler, quoted in the statement.
"Our captain fulfilled his duty of rescue at sea in an exemplary manner. He saw emergency situations at sea and carried out rescues in complete safety. EU states can draw inspiration from this," he added.
Italy is one of the main entry points into Europe for migrants from North Africa, mainly from Tunisia and Libya, from where departures are now far higher than in previous years.
Around 15,000 people have arrived on the Italian coast since January, almost three times more than at the same time in 2020, according to the Interior Ministry.