Europe’s leading anti-torture watchdog has called on the government to process asylum claims in the UK rather than sending people to Rwanda because of the risk they may be exposed to human rights abuses there.

In a report published on Thursday, the Council of Europe’s committee for the prevention of torture and inhuman or degrading treatment or punishment raises a litany of concerns after an 11-day visit to the UK in March and April last year.

The report warns that the UK’s Illegal Migration Act, which allows asylum claims to be determined in Rwanda rather than in the UK, and the migration and economic development partnership between UK and Rwanda “raise multiple concerns over the treatment of vulnerable persons” and warns that they may be subject to torture or inhuman, degrading treatment if they are sent to Rwanda.

The purpose of the committee’s ad hoc visit was to examine immigration detention conditions in the UK at a time when the Home Office plans to significantly increase the number of asylum seekers it locks up before the forced removals it hopes to implement to Rwanda.

The current number of detention spaces is 2,245 and the government wants to add another 1,000 places.

The European court of human rights relies on the committee’s findings when ruling on relevant cases – the government’s first planned deportation flight to Rwanda, which was due to take off on 14 June 2022, was halted after an intervention from the ECHR, and the court may consider the Rwanda policy in the future.

The report raises concerns about the UK’s practice of indefinite immigration detention and about keeping some immigration detainees with criminal convictions in prisons rather than moving them to detention centres after the conclusion of their sentences.

It warns that the Illegal Migration Act, a bill at the time of the committee’s visit, makes the removal of those arriving without a valid visa easier “by stripping away a series of fundamental safeguards”.

It urges the UK government not to use “inflammatory or derogatory language” when referring to foreign nationals arriving in the UK after hazardous journeys.

The report raises concerns about detainees, including those who are victims of torture or at risk of suicide, whose vulnerabilities are highlighted in a process in immigration detention known as rule 35. Once bail has been granted to detainees, their rule 35 categorisation is no longer counted in official statistics, even if they remain in detention centres for several months longer. This means “official statistics cannot be considered accurate” and people deemed unfit for detention continue to be detained.

The committee raised concerns about the treatment of some detainees, including a man who had initially protested but was then compliant. Four guards in full body armour surrounded him and he was handcuffed.

The Illegal Migration Act allows children to be detained for possible deportation to Rwanda, a controversial practice previously ended in the UK but now reintroduced. In response to concerns about this by the committee, UK government officials confirmed that children may be detained and that “where practicably possible” children’s needs in detention will be met.

In response to the report, UK officials said: “The UK government does not recognise much of the content of this report and feels it does not accurately reflect the important work we undertake to ensure the safety and wellbeing of those in our care.”

In response to the delegation’s call to process asylum claims in the UK rather than in Rwanda, officials said they were satisfied the Rwanda plan was in line with international refugee and human rights law and that the courts had recently found the principle of removal to a safe third country lawful.

In November 2023 the UK supreme court ruled that the Rwanda scheme was unlawful and that Rwanda was not a safe country to which to send asylum seekers.