The first of four Algerian terror suspects who have agreed to be deported has left the UK, the Home Office says.
The men felt their only option was to return home, lawyers said
The government intended to deport the other men who have also withdrawn their appeals against deportation as soon as possible, a spokeswoman added.
The men are among a group of 27 foreigners held because of fears that they are a threat to national security.
Some of the men, who cannot be named due to a court order, have been held without trial for more than four years.
In 2004 their detention under anti-terror legislation was deemed unlawful but they were then held under immigration law.
The Home Office spokeswoman said on Sunday: "An Algerian national who had been identified as representing a threat to the national security of the UK was yesterday deported to Algeria.
"This is not the first time that we have removed suspected international terrorists to Algeria - two were returned in June 2006."
She added that a number of others were appealing against their deportation and their cases were currently before the Special Immigration Appeals Commission.
The men are all suspected terrorists and some are believed to have connections to radical Algerian terror cells.
Human rights campaigners say the men, who cannot be named because of a court order, will face torture on their return.
This has been denied by Algeria and the British government says it has assurances that the men will not be mistreated.
Lawyers said the suspects could no longer bear indefinite detention and felt their only option was to agree to go back home.
Gareth Peirce - whose firm Birnberg Peirce is representing three of the Algerians - said the men had chosen a "quick death there rather than an endless slow death here".
She added: "Each man goes in despair of ever clearing his name.
"All research into the effects of wrongful convictions speaks of the devastating effect of wrongful accusations upon the individuals and their families.
"For these men there have been no convictions, no proper accusations, no knowledge of what is alleged against them and, astonishingly for most, no questioning by police to discover whether untested secret assumptions might be wrong."
A Home Office spokeswoman said: "It remains our view that those detained on strict bail remain a real risk to the national security of this country."
Shami Chakrabarti, director of civil rights group Liberty, said: "The government should be ashamed when people would prefer to face torture rather than indefinite detention without trial.
"Diplomatic assurances are a farce - Algeria has signed international agreements against torture but the reports of ill treatment in that country continue."