With the home secretary outlining his policy on what to do with foreign terror suspects after legal defeat in the House of Lords, BBC News looks at the background.
As home secretary, David Blunkett said the detentions were essential
How many people are being held?
Seventeen people have been held under specialist anti-terrorism powers since the 11 September attacks and twelve are still in detention, mostly at Belmarsh prison in London.
Who can be detained?
Under the Anti-Terrorism, Crime and Security Act (ATCSA), the home secretary can indefinitely detain without charge a foreign terrorist suspect if the individual cannot be deported for other legal reasons.
This was introduced because the government believed there were individuals in the UK who were a potential threat but it could not deport back to regimes known for human rights abuses.
What did the Law Lords rule?
The Law Lords, UK's highest judges, ruled on 16 December that detaining foreign terrorism suspects without trial breaks human rights laws.
The ruling was a blow to the government's anti-terror measures and has constitutional significance.
The nine Law Lords ruled by an eight to one majority in favour of appeals by nine detainees.
The case was heard by a panel of nine Law Lords rather than the usual five because of the constitutional importance of the challenge.
The Law Lords said the "draconian" measures were incompatible with European human rights laws and the government must pay the appeal costs.
Why did they make this ruling?
The ruling followed a hearing by the nine Law Lords on 4 October when lawyers for the nine detainees went before the House of Lords to put their case.
The government mounted a vigorous defence, having won its case over several of the detainees at a Court of Appeal hearing in August.
For his part, David Blunkett, home secretary at the time, insisted that the powers remained in reserve to deal with extremely difficult cases and that, to date, he had used them properly.
What evidence is needed to detain people?
We don't always know. David Blunkett, who used these measures as home secretary, said it was often extremely sensitive intelligence material which neither the detainee nor his lawyers is allowed to see.
Under the system for these detainees, special security-vetted lawyers - "special advocates" (SA) - see the evidence and argue its merits against the government legal team.
However, they cannot discuss what they have seen with the suspect - the controversy at the heart of the policy. How, say campaigners, can someone defend themselves if they have not heard the allegations?
The government argues a lot of what is presented in court is of such sensitivity that it would endanger lives or ongoing operations were it to be aired in public. But lawyers opposed to the system say this means a fair hearing is impossible.
Can people appeal against detention?
The Special Immigration Appeals Commission (Siac) is the venue of appeal for foreign nationals facing detention, deportation or exclusion from the UK on grounds of national security.
HM Prison Woodhill: 'Detainee D' released in September
It has the same powers as the High Court and is presided over by senior judges but Siac's hearings and rulings are never fully revealed to the public - or to the appellant themselves - because they include testimony from members of the secret security services which the government says it cannot divulge.
After hearing from the appellant, the home secretary and the SA, the commission issues an open judgement for the appellant and public and a closed ruling, taking in all the factors heard in secret.
If someone loses their appeal, they can only take their case further, theoretically to the House of Lords, on a point of law.
However, in one recent case, the Court of Appeal backed Siac's decision to release a man, saying the home secretary had no legal reasons to object.
What is the argument against the government's detention powers?
Civil rights groups including Amnesty International, the Civil Liberties Trust and Liberty say the powers deny the fundamental human rights of detainees.
Amnesty made written submissions to the Law Lords arguing that the detentions under ATCSA are criminal and violate fair trial rights.
The group also suggests the use of evidence obtained by torture of a third party violates international law.
Lawyers acting for the detainees claimed the government interpreted the rules allowing them to opt out of part of the European Convention on Human Rights too broadly.
They say the Court of Appeal was wrong to see the anti-terror act as a proportionate response to the 11 September attacks.