An EU-funded refugee camp on the Greek island of Samos built to house 640 people is home to 3,745, with unaccompanied children forced to sleep on the floor of windowless and overcrowded containers, an official audit has revealed.

Other children were found to be living in makeshift tents or in derelict buildings on the outskirts of the camp in what the special report from the European court of auditors described as “dire conditions”.

“Seventy-eight unaccompanied minors were in tents or abandoned derelict houses outside the hot spot, in unofficial extensions to the facility,” the auditors said. “Nine unaccompanied girls were sleeping on the floor in a 10 metre sq container next to the police office, with no bathroom or shower.”

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The situation at Greek facilities on Samos and at the island of Lesbos are described as “highly critical in terms of capacity and the situation of unaccompanied minors”, due to a failure of the bloc’s relocation and return scheme for migrants coming to Europe from countries such as Syria.

The auditors’ findings were echoed on Wednesday in appeals made by the mayor of Samos, Georgios Stantzos, who warned of riots due to the “primitive” conditions. Last month, a fire broke out in the camp after a brawl in town between rival groups of Syrians and Afghans.

“People are camping in dry stream beds,” Stantzos told Agence France-Presse. “They are at risk from fires and floods. [Camp residents] organise their own housing, water supply and sanitation in completely primitive ways. We are trying to remain calm but the situation is not manageable ... it gets worse every day. We are past the red line. Any random event ... could lead to terrible results.

At the peak of the migration crisis, EU countries set a target of relocating 160,000 newly arrived migrants. Member states later legally agreed to revise down the plan to relocate 98,256 people.

Of those, only 34,705 – 21,999 from Greece and 12,706 from Italy – were found new homes in the EU. Countries such as Hungary, Poland and the Czech Republic have refused to fulfil their obligations.

The auditors found that relocation schemes in Italy and Greece, which is the focus of their report, had underperformed mainly due to the very low share of potentially eligible migrants registering for relocation. Authorities were said to be unable to identify all potential candidates and successfully channel them towards applying for relocation.

But there were also major systemic problems making the schemes unattractive to applicants. In Greece, people seeking relocation from camps waited an average of 215 days from application to decision last year. A shortage of doctors to assess the vulnerability of applicants was said to be a key factor.

In Italy, the large number of appeals against decisions has led to a backlog. The auditors said it took on average over four years for an asylum application lodged in 2015 to reach the final appeal stage.

The problems have led to unacceptable conditions in the camps, where the NGO Médecins Sans Frontières reported an increase in claims of harassment, sexual assault and other forms of violence. The European court of auditors made their visit to the camps in February.

The European commission has accepted a series of recommendations in the report, including the upgrading of facilities.

However, a formal response published alongside the report added that “the responsibility to establish a sustainable system for unaccompanied minors lies with the Greek authorities, and not with the commission”.

Leo Brincat, a member of the team responsible for the report, called for all EU institutions and member states to step up and avoid passing the buck.