Four men have been held at an immigration removal centre for more than two years as it emerged the average length of detention there has increased, inspectors found.
A report revealed detainees at Brook House Immigration Removal Centre near Gatwick Airport had seen their average length of detention increase from 28 to 48 days.
Inspectors highlighted serious delays in some individual cases, with 23 detainees held for more than a year and four of these detained for more than two years.
The longest detention at the centre was for more than two and a half years, the report by HM Chief Inspector of Prisons Peter Clarke said.
Concerns were raised about the apparent lack of analysis to explain the rise in average length of detention at the G4S-run centre, which held just under 400 men at the time of inspection.
The report, which followed an unannounced inspection last year, said: "Our casework analysis revealed cases of detention being prolonged by delays in immigration decision-making."
Other concerns included the "stark and impersonal" residential units and unsatisfactory sanitary facilities, leading detainees to feel they were being held in prison.
But overall the report said it was an "encouraging" inspection.
Brook House (pictured) was deemed "reasonably good" in all four healthy establishment tests - safety, respect, activities and preparation for removal or release.
Praising staff, Mr Clarke said: "This also marks excellent progress from the standards we were seeing at Brook House when it first opened.
"There is no doubt in my mind that the standards now being observed at the centre are the result of a great deal of hard work by the management and staff."
A Home Office spokesman said: "We are pleased that HM Chief Inspector of Prisons has recognised the improvements made at Brook House since the last inspection, and that excellent progress has been made since it first opened.
"Detention is an important tool that helps us remove those with no right to be in the country and it is vital that this is carried out with dignity and respect. We take the welfare of our detainees very seriously.
"We are considering the contents of the report and the Chief Inspector's recommendations carefully."
Government officials said nobody is detained indefinitely, and there were some who prolonged their detention through attempts to frustrate the removal process.
In some cases, prolonged detention is due to a detainee's failure to provide accurate information on time about their nationality or identity, which was needed to secure travel documents.
Regular reviews of detention are carried out to make sure it remains "lawful and proportionate", officials added.
Campaigners voice concerns over 500 men held at UK immigration removal centre
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.
Dr Charles Campion-Smith, chairman of the group, said: "Many local people are not aware of what is happening on our doorstep.
"Over 400 men, many of whom have committed no offence, are detained behind razor wire in this former prison.
"Their detention is indefinite, they do not know if and when they will be allowed to return to their families or be deported and many are hundreds of miles away from friends and family."
A Home Office spokesman said regular reviews of detention are undertaken to ensure that it remains lawful and proportionate.
"Detention and removal are essential parts of effective immigration controls but it is vital they are carried out with dignity and respect," the spokesman said.
"When people are detained, it is for the minimum time possible and the welfare of those in our care is of the utmost importance.
"No-one is detained indefinitely and the law is clear that the detention power can only be exercised if there is a reasonable prospect of removal within a reasonable time frame.
"Individuals are able to apply for immigration bail and challenge their detention in the courts."
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.
Copyright (c) Press Association Ltd. 2017, All Rights Reserved. Picture (c) Gareth Fuller / PA Wire.