Italian magistrates have accused NGO charity Medecins Sans Frontieres (MSF), which runs the Aquarius migrant rescue boat, of illegally dumping toxic waste at ports in southern Italy - and ordered the ship to be impounded.
The charity has denied any wrongdoing and accused Italy of seeking to criminalise humanitarian search and rescue missions.
MSF tweeted on Tuesday saying it was submitting an appeal to Italian review courts.
The Aquarius has been blocked at the French port of Marseille since September, having failed to find a country willing to register its trips to the southern Mediterranean after its flag was revoked by Panama.
It was not immediately clear if French authorities would impound the boat.
On Tuesday, the Public Prosecutor's office in Catania said it believed crew members on the Aquarius, and its sister ship VOS Prudence, had illegally dumped potentially dangerous medical waste among ordinary rubbish.
They said the dumping took place between January 2017 and May 2018.
It said in a statement 24,000 kilos of suspect waste was handed over to local authorities at several ports in Italy, avoiding costly safe disposal procedures amounting to 460,000 euros.
Waste material included migrants clothes, food and "infectious sanitary waste" it said.
Twenty-four people, including two Italian shipping agents and the Russian captain of the Aquarius, have been placed under investigation, prosecutors said.
MSF has denied the claims and said procedures were always followed when disposing of waste.
“This is an extreme, worrying effort to stop at all costs our search and rescue activity in the sea,” Karline Kleijer, head of emergencies at MSF, said in a statement.
SOS Mediterranee charity, which works alongside MSF, called on France to “show restraint” in the face of the Italian investigation.
Italy has led a crackdown on the Aquarius and denied the boat access to local ports following the arrival of almost 650,000 people from North Africa since 2014.
Interior Minister Matteo Salvini, who has been very vocal with his anti-immigration stance, welcomed Tuesday’s move by Catania magistrates.
“I did well to block the NGO ships. I stopped not only the traffic of illegal immigrants but, from what it seems, also that of toxic waste,” he said on Twitter.
In a subsequent tweet on Tuesday he said "the godsend is over."