Concerns are being raised about the effects of detention on men held at one of the UK's immigration removal centres.
Nearly 500 are detained at The Verne in Portland, Dorset, and campaigners say many have little or no indication about how long they will be there.
The Verne Visitors Group said the indefinite nature of detention caused the detainees distress and was against British principles of justice. The visitors' group, which offers support to detainees, said men were typically held for issues such as expired visas or were failed asylum seekers, while others were criminals awaiting deportation.
A Home Office spokesman said regular reviews of detention are undertaken to ensure that it remains lawful and proportionate.
The Home Office said that of the people leaving detention last year, 64% had been in detention for less than 29 days and 93% for less than four months.
In 2015 62% left detention within 29 days and 92% in less than four months.
Illegal entry, overstaying or obtaining leave to remain by deception are criminal offences under section 24 or section 24A of the Immigration Act 1971.
Persons who require leave to enter or remain but do not have it are liable for removal from the UK under section 10 of the Immigration and Asylum Act 1999.