The Court of Appeal ruling in the case of the Libyan known only as "M" will be seen by opponents of the draconian measures implemented in the wake of the 9/11 attacks as a significant turning of the tide.
Lord Chief Justice Woolf made the ruling at the Court of Appeal
The 37-year-old man is the first of the detainees to win his challenge, initially before the Special Immigration Appeals Commission (Siac) and now in front of the Lord Chief Justice, Lord Woolf.
Hailing it as an important decision, the director of Amnesty International, Kate Allen, said: "It is absolutely essential that people know what they are being accused of and that, if there is evidence, they are brought to court."
Home Secretary David Blunkett had relied on M's admission he was a member of an anti-Gaddafy organisation called the Libyan Islamic Fighting Group and had raised money to support the families of members jailed in other countries.
But, as his lawyer, Gareth Peirce, pointed out, this was not a breach of UK law.
Regarding his release, Ms Peirce said her client's options were limited.
"His only choice is to return to a country where he is likely to be killed," she said.
She also doubted whether there were legal grounds for claiming compensation for unlawful imprisonment.
So, where does the Appeal Court ruling leave the legal machinery introduced in 2001 under the Anti-Terrorism Crime and Security Act ?
Mr Blunkett can point to the assessment of the official watchdog, Lord Carlile, who, only last month, said he was "entirely satisfied" the home secretary was right to certify each of the 14 detainees as suspected international terrorists.
However, Lord Carlile, also argued some of the 14 could be deported without undue risk to their safety and he advocated "suspended detention" - a form of house arrest under intensive surveillance - as an alternative to imprisonment.
But Mr Blunkett is in no mood to compromise on the 2001 Act.
Indeed, he has proposed going even further in the fight against terrorism by lowering the standard of proof required for a conviction.
The Appeal Court ruling will also be a welcome boost for Siac.
Its standing as an effective scrutineer of terrorist detentions was badly dented when its lay member, Sir Brian Barder, resigned because he felt its powers were too circumscribed, not least by the Court of Appeal.
He may well be revising that opinion in the light of this judgment.