Held under immigration laws, these men and women have become known as 'Britain's forgotten prisoners'. Their detention will have been ordered by the Home Office which alone also decides how long they must remain in custody.
Few will get the chance to have their loss of liberty independently assessed by a court.
None will be told how long they are to stay in prison - some have been held for as long as 18 months, yet have committed no offence.
Most are black and many are asylum seekers who, fleeing torture and oppression and hoping for refuge in the UK, find themselves incarcerated instead. At yesterday's launch of a report which claims the Home Office is making excessive and unfair use of prison as a means of immigration control and deterrence, their plight was described yesterday as 'one of the gravest scandals in British society'.
Claude Moraes, director of the Joint Council for the Welfare of Immigrants, which produced the report, said: 'No other European Community country uses these kinds of powers or detains people at this level.'
Further, the report reveals that many detainees are held in appalling conditions. Some have been held in police cells - not designed for stays of more than a few hours.
Some have been held in overcrowded local prisons, like Pentonville, where they are confined to their cells for up to 23 hours a day.
But according to Mark Ashford, author of the report, one of the worst effects on detainees is the uncertainty over their futures. 'They are living literally on a knife edge. They are never given any explanations. They never know if they are to be shifted out, sent back to their country, and sent for trial.'
For many the ordeal proves too much. While some detainees will be students and visitors seeking to enter the UK and some who have been resident for some years but are then accused of breaching immigration rules, the most vulnerable are asylum seekers. Since 1987 four asylum-seekers have committed suicide and others have made serious attempts.
Doctors from the Medical Foundation for the Care of Victims and Torture quoted in the report said that prison conditions 'can inherently stimulate refugees to relive their experiences (so-called flashbacks) which are well recognised in survivors of torture'.
In the first six months of 1991, they saw 150 detainees who had suffered a history of torture. They concluded that imprisonment imposes 'unnecessary cruelty and degradation on people already distressed by torture and loss'. The JCWI report calls for public scrutiny of immigration policy and practice because . . . 'it concerns the fundamental right of liberty of the individual; it raises the question of racial discrimination and it involves criminalising people for actions not obviously detrimental to society'.
It calls for detention only in exceptional circumstances with the immigration service being compelled to give written and specific reasons in each case. This should be automatically subject to independent and regular reviews.
However, a Home Office spokesman said yesterday: 'Only a very small number of people are detained. In 1991, 8.1 million people came from outside the EC - 45,000 were asylum seekers.
'In that period we detained and removed 4,500 people under our enforcement programme. Eighty-five per cent of people detained are held for less than a fortnight.'
Detention Without Trial; Mark Ashford, JCWI, 115, Old Street, London EC1V 9JR; pounds 4.99.