Inspectors have discovered high levels of self-harm, violence and use of force at an immigration detention centre in Lincolnshire where one detainee had been held for more than two years.

Uncertainty about detainees’ immigration status and the potential for long-term detention continue to cause frustration at Morton Hall immigration removal centre (IRC), a report by Her Majesty’s Inspectorate of Prisons (HMIP) says.

Inspectors found cases in which the Home Office had decided to continue to detain individuals despite accepting they had been tortured, the report says.

The UK has one of the largest immigration detention systems in Europe and is the only country in the region without a statutory time limit on length of detention.

Inspectors found that Morton Hall IRC, which is operated by the prison service on behalf of the Home Office and holds adult men subject to immigration control, had improved since the last inspection in 2016 but most of the safety concerns remained.

Detainees held for lengthy periods were often detained because of documentation problems, a lack of suitable accommodation or casework inefficiencies, the report says.

At the time of the inspection in November last year there were 241 detainees. One had been held for more than two years and another had been awaiting an asylum decision for 11 months.

Doctors had submitted 173 reports to the Home Office in the previous six months to raise concerns that a detainee’s health was being affected by continued detention, the report says.

Inspectors reviewed 10 of the reports and found that references to injuries or use of weapons suggesting torture were not explored sufficiently.

“The infliction of torture was accepted by the Home Office in nine cases,” the report says. “In six of them, detention was maintained because negative immigration factors were considered to outweigh the risks to the detainee.”

There were several clear indications of the vulnerability of the population, inspectors said, and they called for managers to investigate an increase in the number of self-harm incidents and the number of individuals who self-harm.

There had been two self-inflicted deaths since the previous inspection, the most recent of which was in 2017. There had been 113 self-harm incidents in the previous six months, more than at the previous inspection, the report says.

Levels of violence and use of force were still too high but there were few serious incidents, inspectors found.

The report highlights shortcomings in the way that age dispute cases were managed. In the previous year, two people claiming to be children were detained without a sufficiently thorough age assessment having been undertaken.

Morton Hall was previously used as a regular prison. Inspectors said the physical environment remained too prison-like, with cellular accommodation and a large amount of razor wire.

Locking detainees in cells at night remained an excessive restriction, the report says, and an 8.30pm lockup time was too early.

Since the previous inspection, the total number of detainees had declined by about a third while staffing levels had remained the same.

A Home Office spokesperson said: “We have made significant improvements to ensure those detained are treated with dignity, respect and with the utmost importance placed on their health and wellbeing.

“This includes additional safeguards and support for vulnerable people, such as robust monitoring and care for those at risk of self-harm, as well as an Adults at Risk policy which makes sure vulnerable individuals are only detained when the immigration factors outweigh the risk of harm.”