The High Court has ruled that US intelligence documents containing details of the alleged torture of a former UK resident can be released.
Ethiopian-born Binyam Mohamed, 30, who spent four years in Guantanamo Bay, claims British authorities colluded in his torture while in Pakistan.
High Court judges were previously blocked from making the details public.
The Foreign Office denies allegations of collusion and says it will appeal against the court's judgement.
'Inviolable principle'
In a statement, Foreign Secretary David Miliband said the UK government was "deeply disappointed" at the ruling.
"The issues at stake are simple, but profound. They go to the heart of the efforts made to defend the security of the citizens of this country," he said.
"We only share British intelligence with other countries on the basis that they will not disclose that intelligence without our express permission. The same inviolable principle applies to foreign intelligence shared with us."
Mr Miliband said the government stood "firmly against torture and cruel, inhuman and degrading treatment or punishment", but he vowed to continue to challenge the court's ruling "in the strongest possible terms".
"I am determined that the vigour with which we fight this case will maintain the confidence of and send a clear message to all our intelligence partners across the world," he said.
"The United Kingdom will protect the information that you share with us and uphold the principle that it is for you, not us or our courts, to decide if and when to release such material in to the public domain."
The US also denies any allegations of torture concerning Mr Mohamed.
http://news.bbc.co.uk/1/hi/uk/8311075.stm