The move came after almost 10,000 migrants reached the southern Italian island of Lampedusa last week, dealing a blow to the credibility of Prime Minister Giorgia Meloni who won office last year vowing to curb illegal immigration.
Meloni said at the start of a cabinet meeting on the situation that migrants awaiting repatriation should be detained for an initial six months, extendable to up to 18, up from three months now.
"That will be all the time needed not only to make the necessary assessments but also to proceed with the repatriation of those who do not qualify for international protection," Meloni said in her introductory speech.
Government sources said the cabinet approved that measure shortly afterwards, as well as the creation of more detention centres in remote areas.
Meloni said Italy needed to increase the capacity of such facilities as they had been weakened by "years of immigrationist policies".
Under Italian law, migrants facing repatriation can be held if they cannot be immediately expelled.
Officials say a majority of migrants head to Italy for economic reasons and are therefore not eligible for asylum.
Past efforts to hold migrants have largely failed, with those detained repeatedly breaking out of centres and often heading straight to wealthier northern European countries.
Meloni visited Lampedusa on Sunday with European Commission president Ursula von der Leyen, who promised a 10-point EU action plan, but the measures resembled previous initiatives that have failed to make much impact.
An agreement struck in July between the European Union and Tunisia, from where many of the migrants set sail, has yet to take effect.
Almost 130,000 migrants have arrived in Italy so far this year, according to government data - nearly double the figure for the same period of 2022.
The migrants have come from countries including Guinea, Ivory Coast, Tunisia, Egypt, Burkina Faso, Bangladesh and Pakistan.
The government's latest move to stem the migrant flow was condemned by the opposition and rights groups.
The Italian Coalition for Civil Rights and Liberties (CILD) described the detention centres as "black holes" where serious violations of fundamental rights take place, adding that they are expensive and inefficient.
In April, the Italian parliament approved measures to create new migrant centres for people waiting to hear the outcome of asylum applications, as well as more detention facilities for those facing expulsion.
As part of the package, it set aside about 20 million euros ($A33 million) to fund it over a two-year period.
