The last 12 asylum seekers being held in a new Italian migration hub in Albania must be transferred to Italy, a court has ruled, in a heavy blow to a controversial deal between the far-right Rome government and Tirana aimed at curbing migrant arrivals.

The decision casts further doubt on the feasibility and legality of plans by the EU, discussed on Thursday, to explore ways of establishing migrant processing and detention centres outside the bloc as part of a new hardline approach to migration.

The Italian judges’ ruling on Friday means the Rome government’s new facility has essentially been emptied after four of the first group of 16 asylum seekers to arrive at the processing centre were immediately sent back to Italy on Thursday.

Brothers of Italy, the Italian prime minister, Giorgia Meloni’s party, angrily condemned the decision on social media, blaming “politicised magistrates” who “would like to abolish Italy’s borders. We will not allow it.”

Matteo Piantedosi, the interior minister, said: “We will appeal all the way to the court of cassation. We will continue with what Italy is achieving in Albania, and beyond, it will become European law.”

The 16, all of whom the Italian government argue should ultimately be returned to their “safe” home countries of Egypt and Bangladesh, arrived at the Albanian port of Shëngjin from the Italian island of Lampedusa aboard a military vessel on Wednesday.

Under the deal, signed by the far-right prime minister, Meloni, and her Albanian counterpart, Edi Rama, men intercepted in international waters crossing from Africa to Europe will be held at the centre while their claims are processed.

The scheme, which could process up to 3,000 men a month, excludes women, children and vulnerable individuals, who will be taken to Italy. Of the first four men returned to Italy, two were thought to be under-age and two deemed vulnerable.

The remaining 12 were considered by the judges in Rome to be at risk of violence if they were deported to their home countries, in a decision that in effect upheld a 4 October ruling by the European court of justice (ECJ).

Only migrants coming from a list of 22 nations Italy classified as “safe” can be sent to Albania. Egypt and Bangladesh are among them, but the ECJ ruled that a country outside the bloc could not be declared safe unless its entire territory was deemed safe.

The judge Luciana Sangiovanni said: “The rejection of the individuals’ detention in structures in Albania equated to Italian border or transit zones … is due to the impossibility of recognising the states of origin of the detained individuals as ‘safe countries’.”

Opposition parties and national newspapers in Italy said the initiative, which will cost about €1bn (£830m) over five years, was already a failure, noting that the government had spent €250,000 transporting the 16 men to Albania on a military vessel.

The Democratic party said the plan had failed and Meloni should apologise, while the Europe party demanded Piantedosi’s resignation.

A network of NGOs representing 160 organisations that support undocumented people has described the Italy-Albanian deal as “inhumane, absurd and a costly system that breaches international human rights obligations”.

Michele LeVoy, of the Platform for International Cooperation on Undocumented Migrants, or Picum, said the network was “appalled” at growing support among EU member states and the European Commission for offshore migrant hubs.

“Aside from being a logistical and financial nonsense, it’s a cruel system that breaches international and EU law and puts people at risk of being abused with no clear options to get justice and remedies,” LeVoy said in a statement.

At a summit in Brussels on Thursday, EU leaders discussed setting up “return hubs” – processing and detention centres – in countries outside the bloc and the commission president, Ursula von der Leyen, said talks on how they might work would continue.

The summit’s final statement reflected the bloc’s tough new mood on migration, calling for “determined action at all levels to facilitate, increase and speed up returns from the EU using all relevant EU policies, instruments and tools”.

Activists and researchers have repeatedly questioned whether, compared with a well-funded, EU-based asylum system, offshore hubs or “migrant hotspots” could ever be deemed humane, effective, or even legal under international law.

Besides Albania’s deal with Italy and a small-scale agreement between Denmark and Kosovo, it is in any case unclear which non-EU countries might be willing to host such centres. Some diplomats suspect that for this reason alone, the idea may be a non-starter.

Meloni said after the summit that there were “many countries looking at the Albania model” and several other far-right leaders praised what the Dutch prime minister, Dick Schoof, described as “a different mood in Europe”.

Others, however, were cautious, questioning the expense, complexity and effectiveness of an “offshore” model.

EU irregular immigration has plummeted since the 2015 migration crisis and is down more than 40% this year compared with 2023, but the bloc’s tough approach reflects a string of electoral success by far-right, anti-immigration parties.