The Italian Deputy Prime Minister, Matteo Salvini, on Thursday launched a crackdown on the German charity Sea Watch, whose ship rescued 52 migrants off the Libyan coast the day before.
Mr Salvini, who is also Minister of Interior, leads the powerful right-wing League party in the ruling coalition.
He issued a decree ordering law enforcement authorities to take all necessary measures to prevent the entry into, or transit through, Italian waters by the Sea Watch 3.
The charity responded on Twitter: "Sea Watch won't disembark survivors in Libya. Tripoli is not a port of safety.
"It is a crime to forcibly return rescued people to a country at war, where they face unlawful imprisonment and torture. Italy promoting these atrocities and the EU being complicit is outrageous."
On Tuesday, Mr Salvini said the decree would bring fines of up to €50,000 (Dh207,160) for the captain, owner and operator of a vessel "entering Italian territorial waters without authorisation".
Mr Salvini's popularity soared in the past year with a hard line against migrants that has included closing ports to rescue vessels.
He has attributed a decline in the number of deaths at sea to a deal with the Libyan coastguard to stop people heading to European shores.
Mr Salvini says those setting sail from Libya to seek safety in Europe should be returned to the crisis-hit country.
The order is contentious under international law and charity migrant rescuers have repeatedly refused to follow it.
More than 12,000 people have died since 2014 trying to flee Libya to Europe by what the UN refugee agency calls the "world's deadliest sea crossing".
The decree still has to go before Italy's Parliament where the coalition government holds a comfortable majority.