Prosecutors in Sicily have placed Italy’s prime minister and three senior members of his government under investigation for false imprisonment over the detention of 47 migrants who were prevented from disembarking from an NGO rescue ship.

Giuseppe Conte, deputy prime ministers Matteo Salvini and Luigi Di Maio, and Italy’s minister of infrastructure, Danilo Toninelli, face charges for refusing to allow migrants to leave the Sea-Watch boat, which rescued them off the coast of Libya on 19 January.

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The 47 people on board were forced to wait off Sicily for more than a week after the Sea-Watch 3 was denied the right to dock in Palermo, drawing the anger of the UN and prompting an emergency appeal to the European court of human rights by the German NGO that operates the boat.

Salvini has repeatedly declared Italian waters closed to NGO rescue vessels. Several boats have been left stranded at sea due to his hardline approach, which is partly designed to force other parts of Europe to take more asylum seekers.

“I’ve been placed again under investigation,” Salvini, who is also Italy’s interior minister, tweeted on Monday. “However, with me the ports will remain unavailably closed and sealed to the traffickers in human beings.”

In January, an Italian court ruled that Salvini should be tried for the kidnapping of 177 asylum seekers he prevented from disembarking the Italian coastguard ship Ubaldo Diciotti in August last year. But in March senators ruled that Salvini should not be stripped of his parliamentary immunity and the case fell apart.

Before the vote, Salvini’s coalition partner, Luigi Di Maio, who heads the Five Star Movement (M5S), supported his actions, arguing the decision was taken by the whole government. Di Maio filed a document with the prime minister, Giuseppe Conte, formally defending him. “If Salvini is responsible for the seizure [of the migrant boat] then the whole government is responsible,” it said.

The ultimate decision rests with the court of ministers and the senate. Magistrates have three months to decide if Di Maio, Salvini, Toninelli and Conte should face trial – and then the senate will vote again on stripping them of immunity.

“The detention of migrants for propaganda purposes cannot remain unjust yet again. People fleeing Libya must be rescued and protected, not exploited,” a Sea-Watch spokesperson said. “Everything else must be discussed on land, not at sea. We are ready to testify what it was like to have to explain to the people on board that a country in peace does not want them.”