Yarl's Wood detention centre was visited by the commissioner this year
Changes to Britain's asylum and immigration controls could breach human rights, a European watchdog has warned.
A report from the Council of Europe said UK officials should reconsider new fast-track processing procedures.
It also recommended "drastically limiting" the policy of "administrative detention" of migrants, and proposed a maximum time limit for detentions.
The Home Office said the UK would ensure "speedy decision making does not mean rushed decision making".
The council's commissioner for human rights, Thomas Hammarberg, visited UK immigration centres, including Colnbrook and Yarl's Wood Immigration Removal Centres, in February and April.
He also talked to NGOs and authorities including Home Office Minister Liam Byrne.
His report said: "Improvements must be introduced to strengthen effective respect for the rights of asylum-seekers and immigrants in the UK.
"The UK authorities should consider regulating the so-called 'Detained Fast Track' by introducing special legislation fully in compliance with the standards laid down by the European Convention on Human Rights.
"This type of detention should in particular be forbidden for vulnerable persons, such as unaccompanied minors, for whom alternative measures should be provided."
While he praised some efforts to improve efficiency, Mr Hammarberg said human rights could be at threat because of the complexities of refugee law and protection.
Freedom from torture and ill-treatment is a fundamental pillar of European societies
Council of Europe commissioner for human rights
The report stated: "The commissioner is fully cognisant of the migration flow pressure exerted on the UK, similar to other Council of Europe member states.
"This complex, international and social phenomenon must be dealt with in a manner which is not only efficient but also effectively respects human rights principles."
Time limit rejected
The report was not encouraging of plans to expand immigration detention facilities, saying this was likely to increase the practice of "administrative detention".
Mr Hammarberg recommended a maximum time limit for detentions be introduced into domestic law, while immigration staff should be subject to "on-going education" in human rights protection.
The Home Office response, included in the report, rejected the suggestion of including a time limit in law because it said there were already safeguards in place.
The report expressed concern about the welfare of migrant children during vetting, insisting: "Further efforts are also needed to provide alternative solutions to detention for families and children."
The Home Office defended its detention, saying: "The government has no wish to detain people any longer than is required and this is particularly true in the cases of families with children.
"However, there are occasions where detention is prolonged as a consequence of attempts to frustrate the removal process. The courts have upheld that continued detention in such circumstances remains lawful."
Mr Hammarberg strongly criticised the general UK practice of forced returns on the basis of diplomatic assurances, usually sought from countries which the report said had long-standing, proven records of torture and ill-treatment.
"Freedom from torture and ill-treatment is a fundamental pillar of European societies," he said.
"It is absolutely crucial that the (UK) authorities respect this principle and ensure effective protection of the returnees' safety and dignity, also by monitoring their reception in the country of origin."
The Home Office said the assurances had to "satisfy ourselves and the courts" that removal met international and national obligations.
"Even where we have assurances in place, we will not deport someone in the unlikely scenario that there were substantial reasons for believing that there was still a real risk of torture or other inhuman or degrading treatment or punishment or that the death penalty would apply," the Home Office said.