At the end of December 2013 2,796 people were being held in detention centres in the UK, a 4% rise from December 2012. In addition, migrants are routinely detained under immigration powers in prisons. For example, 790 migrants were kept in prisons solely under immigration powers as of 3 June 2014.

The Immigration Act 1971 was the first law to include the power to detain immigrants. Detainees are held by the administrative authority of Home Office officials and there is no time limit on how long individuals can be detained under these powers. Guidance suggests that detention pending removal from the UK should only take place when removal is ‘imminent’. However, Home Office statistics show that people are held in detention for over a year. During the year ending March 2014, 29,801 people left detention. Of these, 18,115 (61%) had been in detention for less than 29 days, 5,703 (19%) for between 29 days and two months and 4,127 (14%) for between two and four months. Of the 1,856 (6%) remaining, 175 had been in detention for between one and two years and 39 for two years or longer.

The Home Office has 10 ‘Immigration Removal Centres’, as well as a prison that exclusively holds detainees. Current policy outlines that detention is normally used in the following circumstances: initially to establish a person’s identity or basis of a claim; to effect removal; and where there is reason to believe that the person will fail to comply with any conditions attached to the grant of temporary admission or release. Migrants can also be detained while awaiting a decision by the Home Office on whether or not to grant them leave to enter the UK. Some specific categories of people who can be detained are outlined below:

1. Asylum Seekers (including new arrivals) awaiting outcomes of their applications

Asylum Seekers: Asylum seekers are those who have requested protection as a refugee. In 2012, 48% of immigration detainees were asylum seekers.

Detained Fast Track: If an immigration official believes that a person’s asylum case is ‘straightforward’ their claim for asylum can be processed through the ‘Detained Fast Track’ system. The Fast Track process usually takes several weeks, and imposes very tight deadlines for appeals. Asylum applications considered likely to be ‘clearly unfounded’ can be routed onto the ‘Detained Non Suspensive Appeal’. This means that the Home Office can ‘certify’ their application, meaning that at the end of their 10-14 day asylum process the person has no right of appeal in the UK to an independent court or tribunal.

2. Refused asylum-seekers and other migrants pending removal

Unsuccessful asylum applicants: Many detainees are asylum-seekers whose claims have been refused and who are detained for removal.

Newly Arrived Migrants: Newly arrived migrants who have been refused permission to enter the UK can be detained.

Visa overstayers: Overstayers are those who have previously have had a visa allowing them to be in the UK, but this has then run out. Some may have unsuccessfully applied for this to be extended.

Breach of conditions of visa: Those who have not complied with the terms of their visa can have their visa cancelled and be detained.

3. People considered likely to fail to comply with any conditions attached to the grant of temporary admission or release 

Concerns a person might abscond: If the government has grounds to believe that an asylum seeker or migrant might abscond or not abide by the conditions for entry then the asylum seeker or migrant can be detained.

4. Foreign national ex-offenders pending or appealing against deportation

Foreign National Ex-Offenders: Many ex-offenders are detained in prisons. Recent figures also show that there are 790 detainees held in prisons for immigration purposes . These are people who have completed their sentence but are kept in a prison on grounds that their immigration status is not guaranteed.