Italy has called for the seizure of the Aquarius migrant rescue ship, alleging it illegally dumped 24 tonnes of potentially toxic waste in its ports.
One of two charities that lease the Aquarius, Doctors Without Borders (MSF), condemned the measure as "unfounded and sinister".
The Aquarius is involved in a long-running feud with Italy over its rescue operation in the Mediterranean.
It has rescued thousands of migrants but is currently docked in Marseille.
It has been stuck in the French port for months since Panama revoked its registration - citing intense political pressure from Italian authorities. It has since had trouble maintaining stable registration status, and a de-flagged vessel cannot legally set sail.
MSF says this disagreement centres around waste management of items such as food leftovers, clothes, and medical waste.
Prosecutors in Sicily say 24 tonnes of potentially hazardous waste were dumped over a period of two and a half years in 11 ports, of which five are in Sicily.