A navy doctor said the migrants had “likely suffocated” in the tiny space, and “advised against removing the bodies” as it was not yet clear whether there were poisonous gases in the hold which might affect others.

“Another 30 dead in a boat. Another 30 deaths on the consciences of those who defend Mare Lorum,” said Matteo Salvini, head of the anti-immigration Northern League party, in an ironic reference to the country’s “Mare Nostrum” (“Our Sea”) operation to rescue boat immigrants.

The League has warned Prime Minister Matteo Renzi’s government that plucking asylum seekers and immigrants to safety from their rickety boats only encourages more people to set out across the Mediterranean for Europe.

“Renzi and Alfano’s shirts are soaked in blood. Stop the departures, help them in their own countries, immediately!” he said in a post on Facebook, pointing the finger not only at the centre-left leader but also at Angelino Alfano, Italy’s interior minister.

Maurizio Gasparri, the Senate leader of Silvio Berlusconi’s Forza Italia party, blasted Renzi’s “demented” navy operation and the “thousands of landings, deaths, tragedy, chaos. We cannot go on like this.”

In a statement, the navy said some of the 30 migrants may have drowned rather than suffocated, though it would not confirm whether there was water in the hold or how much.

– ‘We cannot face this emergency alone’ –

It is not the first time Italian rescuers have found migrants dead on the overcrowded boats but never before was there such a large number.

The boat is being towed by the Italian navy and is expected to arrive Tuesday in Pozzallo on the southeast coast of Sicily.

“We cannot face this emergency alone,” Luigi Ammatuna, the mayor of Pozzallo, told ANSA news agency.

“The only two refrigerated rooms in the cemetery are occupied by the bodies of migrants. Where will we put the 30 victims of this atrocious tragedy?”

He said it was “impossible to take in the 900 or so migrants who are about to arrive” because “the reception centres in the area are full”.

Three military ships carrying over 1,000 migrants each are expected to arrive in ports in southern Italy later Monday and Tuesday, the navy said.

Coast guard vessels and cargo ships carrying hundreds of others are set to arrive in Sicily, bringing the number rescued over the weekend to almost 5.500.

The number of migrant arrivals has now soared past the record 63,000 set in 2011 during the Arab Spring uprisings.

Italy has long borne the brunt of refugees making the perilous crossing from north Africa to Europe, but EU border agency Frontex says there has been a significant rise in numbers in recent months.

A series of tragedies has struck in the last few weeks, with ten people drowning and 39 having to be rescued after their boat sank off the Libyan coast earlier in June.

“No-one can dream that these deaths will end while the journeys continue. They are journeys of hope, but increasingly end up as journeys of death,” the archbishop of Agrigento in Sicily, Francesco Montenegro, told Radio Vatican.

Alfano has called for the rescue operation to become a European initiative amid reports of thousands of migrants waiting in Libya to make the trip during the next few weeks.