A human rights report on Thursday says the 2001 Anti-Terrorism Crime and Security Act permits the indefinite detention of non-UK citizens without charge or trial.
Amnesty International charges that the legislation fails to meet international standards for a fair trial, permits indefinite detention on the basis of secret "evidence", and allows the use of "evidence" extracted under torture.
There are currently 14 people held under this legislation, all of them accused of being a threat to national security as "suspected international terrorists".
Six of the detainees will have been in detention for two years on 19 December.
September 11 attacks
The legislation was introduced after the September 11 attacks on the United States which killed about 3000 people.
"The Act is discriminatory - there is one set of rules for British citizens and another for nationals of other countries," Amnesty said.
"It effectively allows non-nationals to be treated as if they have been charged with a criminal offence, convicted without a trial, and sentenced to an open-ended term of imprisonment. In no respect can this be considered just."
"If there is sufficient evidence to warrant holding these individuals indefinitely they should be charged and tried in proceedings which meet international fair trial standards. Otherwise they should be released"
The rights organisation said the Special Immigration Appeals Commission (SIAC), which is dealing with the detainees' cases, had effectively denied them the presumption of innocence.
Amnesty added the burden of proof used to detain the accused was lower than that of even a civil case.
Burden of proof
"These individuals face indefinite detention on the basis of a lower standard of proof than would be necessary in a civil court case to recover damages following a car accident.
"What is more, they can be held indefinitely on the basis of secret 'evidence'. Evidence that neither they nor their legal representatives can access and challenge."
Amnesty said it was concerned SIAC had also ruled that "evidence" extracted by torturing was not only admissible, but might also be relied upon by SIAC in reaching its judgments.
"By indicating that they are prepared to rely on evidence extracted under torture, the UK legal process has effectively given a green light to torturers. Using evidence tainted by allegations of torture is contrary to any notion of justice and respect for the law.
"If there is sufficient evidence to warrant holding these individuals indefinitely they should be charged and tried in proceedings which meet international fair trial standards. Otherwise they should be released."
Home Office rejection
However, the UK Home Office has rejected Amnesty's accusations.
A spokesman said David Blunkett, the UK home secretary, had been independently judged to have used his powers under the Act fairly and proportionately.
He said: "There have only been 16 foreign nationals detained under this Act and all of them have been given the right to leave the country at any time they want. Two have already chosen to leave the UK.
"Don't forget, all of these people have the right of appeal as well. So it's clearly untrue to say that they are being held indefinately without charge or trial."
"This has nothing to to with the home secretary's whim or decisions by unaccountable agencies. These people are being held because they are a threat to national security and if certain information was released that would endanger national security"
UK Home Office spokesman
And the spokesman said the detainees could not be tried in an open court because they were a threat to national security.
"This has nothing to do with the home secretary's whim or decisions by unaccountable agencies. These people are being held because they are a threat to national security and if certain information was released that would endanger national security.
"It is normal that foreign nationals are treated somewhat differently to home nationals - that is the same all over the world. This is an immigration matter, after all."
However, the spokesman refused to be drawn on Amnesty's accusation that the legilsation had given the green light to torture.
He said: "Of course we don't condone torture and these practices have certainly not gone on. But as for the act condoning torture, you will have to ask Amnesty about that."
Amnesty's report comes after a string of arrests by British police of alleged "terror suspects".
Four men from West London were recently detained in the capital for one week and then released without charge.
Another four men and two women were detained in Cambridge, and four others near Birmingham in the West Midlands.
Since the September 11 attacks on New York and Washington, 529 people had been arrested under emergency terrorism legislation in the UK. Seventy-seven were charged, but just two were convicted.