Over seven years, some 1,023 foreign prisoners were released from UK prisons when they should have been considered for deportation. The government and police are still trying to trace many of them.
How many of the foreign criminals are still at large?
Eighty-five of the 186 serious foreign offenders are still free.
But the Home Office says 27 of the 37 "most serious" offenders, were in jail, including four murderers, and two were thought to be dead.
Of the 149 other "more serious" offenders, 66 are now under control and six have been deported, it says.
In total, the Home Office says it has now started considering deportation in 999 of the 1,019 cases.
How many of them have reoffended since being freed?
The Home Office says eight of the 37 most serious offenders have committed more crimes, but not involving violence or a sexual element.
Thirteen of the 149 more serious offenders have committed crimes involving violence or a sexual element since their release. Earlier figures suggested another 30 have reoffended with other crimes.
What should have happened?
Foreign citizens face the prospect of deportation from the UK if they are convicted of committing a crime that carries a sentence of imprisonment.
The Prison Service is expected to notify the Home Office's Immigration and Nationality Directorate (IND) that a foreign inmate subject to possible deportation is being held before they are released.
In March 2005 ministers circulated a document entitled Prison Service Order 6000 to all prison governors in an attempt to tighten up this process.
Under the order, governors were told to begin talks over deportation at least 26 weeks ahead of offenders' potential release.
With higher-risk offenders, the process should start 45 weeks before potential release date.
The document also highlighted the possibility of such cases being overlooked and supplied a hotline number for governors to get in touch with the IND's senior lawyers.
Do courts recommend deportation?
In some cases a judge recommends at the time of sentencing that a criminal should be deported.
But such a recommendation will usually be made by a court only where a foreign defendant is convicted of serious offences.
In cases of murder or rape, for example, a judge will often recommend a foreign national be removed from the UK on completion of their sentence.
In the current row, about 160 of the prisoners were subject to specific orders from courts recommending their removal.
What happens if no recommendation is made in court?
The Criminal Justice Act 2003 introduced guidelines on dealing with cases where no court order was made.
Under the act, prisoners from outside the European Economic Area (EU countries plus Iceland, Norway and Lichtenstein) should be considered for deportation if they are sentenced to at least a year in jail.
Those from within the EEA have to be sentenced to at least two years' imprisonment to be considered for deportation.
But deportation within the EEA depends on the home secretary being able to prove that the individual presents a serious threat to the fundamental interests of society - someone who is likely to commit a serious crime in the future.
A foreign citizen should also be considered for deportation if convicted of three lesser offences.
Who makes the decisions?
Civil servants at the Immigration and Nationality Directorate review the cases.
The home secretary's office will need to approve any recommendation before the actual deportation takes place.
A prisoner does have a right to appeal against the decision.
What criteria are used?
The seriousness of the offence and an individual's potential for re-offending will be considered. The age of the person and their ties to the UK will also be taken into account.
Officials will see whether there are any compassionate grounds for the person to remain, as well as considering the UK's obligations under the Human Rights Act.
Will a prisoner ever be freed before a deportation decision is reached?
If a decision has not been made before release date, a foreign prisoner may be held in an immigration detention centre.
However, detention depends on an assessment of the prisoner's risk and the likelihood of them absconding.
A prisoner could be freed pending a deportation decision, but may be asked to wear an electronic tag or made the subject of restriction orders.
What circumstances can prevent a deportation?
There are three key factors that may stop or delay a proposed deportation taking place.
Firstly, there is the target country's willingness to accept the return of an individual in a process known as redocumentation.
Governments have to agree the transfer of criminals, but countries such as China refuse to accept redocumentation of criminals.
Secondly, immigration officials sometimes cannot prove where someone is actually from, particularly if the person facing deportation refuses to co-operate.
Thirdly, under human rights laws nobody can be returned to a country where they will be subject to ill treatment.
For example, a murderer could not be deported to their home country if they would be likely to face execution.
How is the system now going to change?
The man at the top has already gone. Charles Clarke was sacked as home secretary in the recent Cabinet reshuffle and replaced by John Reid.
Mr Reid says he wants to be able to deport automatically any foreign nationals who received "significant" jail sentences.