The control orders have been challenged in court
It has emerged that two terror suspects under control orders to restrict their movements are on the run. The orders have been surrounded by controversy since their introduction in March 2005.
They were introduced to replace emergency laws brought in after the 11 September 2001 attacks in the US.
Those laws had allowed the government to imprison a number of foreign terror suspects without charge or trial.
The men, who became known as the Belmarsh detainees, could not be deported as, under the Human Rights Act, they could not be sent to countries where they might face torture or other ill-treatment.
However, in December 2004, following a legal challenge, the House of Laws ruled such detentions were illegal.
Control orders were created from the need to find another way of dealing with people believed to pose a threat to national security when there is not enough evidence to bring them to trial.
This could be because the evidence against them has been collected by bugging the suspect - and is therefore inadmissible - or because using it could reveal intelligence sources.
Control orders contain conditions restricting the behaviour and movement of the recipient.
In some cases, they effectively allow the authorities to keep suspects under house arrest.
Interim orders, signed by the home secretary, must be referred to a judge within seven days for confirmation.
The control orders can:
Ban possession or use of specified articles or substances and prohibit the use of certain services, such as the internet or phones.
Restrict work or business, restrict association or communication with certain individuals, or other people generally.
Restrict a suspect's place of residence or who is allowed into the premises.
Require a suspect to be at specified places or in a particular area at certain times or days; put a specific 24-hour ban on movements and order the surrender of a passport.
Require a suspect to allow officials to search his home and allow them to remove items from premises for tests.
Require a suspect to be monitored by electronic tagging.
Control orders have proved problematic for the government and have been the subject of legal challenges.
In April, the first British citizen to have a control order imposed on him won a High Court declaration that the laws were "incompatible" with European human rights legislation because they had denied him a fair trial.
On 28 June, the same judge in that case, Mr Justice Sullivan, quashed control orders against six men.
He said the orders were "incompatible" with Article 5 of the European Convention on Human Rights which prevents indefinite detention without trial.
The government appealed against the rulings in July, but the Court of Appeal upheld the judge's decision.
However, it did allow part of Home Secretary John Reid's appeal against the earlier ruling which said another suspect had not been given a fair hearing when put on a control order.
After losing the appeal in August, Mr Reid said he had made new and amended control orders against the six suspects involved. The original orders had kept the men inside for 18 hours a day.
Mr Reid promised to go to the House of Lords to appeal against the deprivation of liberty ruling, because he said he was concerned about its impact on public safety.
Fifteen people are currently under control orders - nine foreign nationals and six British suspects. They are all considered to be a significant threat by the home secretary.
Human rights groups Liberty and Amnesty International are among those who oppose the orders.