The review, to be led by Stephen Shaw the prisons and probation ombudsman until 2010, comes as May prepares to publish a highly critical but more narrowly drawn report on the mental health of detainees, prepared for her by the Tavistock Institute.

She is likely to respond to the review’s damning findings by preparing new guidelines for how mentally ill people must be treated in detention centres, including stressing the primacy of Department of Health advice.

 The system for detaining immigrants and asylum seekers awaiting decisions or deportation has been described as the harshest and most inhumane branch of the UK judicial administration.

The Home Office has faced criticism for years over the welfare of detainees, following incidents of death, self-harm and sexual abuse, as well as repeated court rulings finding Home Office practice unlawful.

The coalition government has also been challenged over the practice of indefinite detention and the sporadic failure to meet its 2010 pledge to end the detention of children.

The implicit acknowledgement by May that the system is in need of reform contrasts with increasingly tough Conservative statements on tightening UK immigration laws. It also marks a break from claims by some junior Home Office ministers that the detention system has improved immeasurably.

The Shaw review, due to take around six months and so reporting after the general election, will look into whether “improvements can be made to safeguard the health and wellbeing of detainees held in immigration removal centres and short-term holding facilities, as well as those being escorted in the UK”.

It will focus especially on pregnant women, elderly people and those with disabilities and mental health issues. Shaw has been told he can visit detention facilities and scrutinise relevant Home Office policies and operational practices.

May said: “Immigration detention is a vital tool in helping ensure those with no right to remain in the UK are returned to their home country. But I take the welfare of those in the government’s care very seriously and I want to ensure the health and wellbeing of all detainees, some of whom may be vulnerable, is safeguarded at all times. We must ensure it treats those we are removing from the UK with an equal sense of fairness.”

The review is expected to look at the appropriateness of Home Office policies on the welfare of vulnerable people in detention, with a focus on safeguarding the mental and physical health of detainees.

There are 11 immigration removal centres (IRCs) in the UK, used to hold migrants and asylum seekers on arrival, pending a decision on their status. Where applications have been rejected, IRCs also hold them prior to deportation.

The UK has more than 4,000 designated immigration detention beds and it is estimated that more than 30,000 people spent time in them in 2013. The government held almost 800 people separately in prisons under immigration legislation in mid-2014.

Campaign groups and lawyers have lambasted the UK for opting out of the European Union returns directive, which sets a detention limit for irregular migrants of 18 months.

Shaw will have to look at the extent to which the Home Office complies with its own rules designed to protect “any detained person whose health is likely to be injuriously affected by continued detention”. On-site doctors are supposed to notify managers as well as the Home Office of such cases, in particular those at risk of suicide and survivors of torture.

Campaign groups say these reports are routinely ignored. A 2011 study found that 9% of such notifications led to the release of a detainee. On six occasions over the past three years, UK courts have ruled that the human rights of mentally ill detainees had been violated.

The Guardian