Judges rejected the government's claim that revealing the information would damage U.S.-British intelligence cooperation.
The information disclosed is a seven-paragraph summary of U.S. intelligence information given to British spies about former detainee Binyam Mohamed's treatment during interrogations by the Americans in May 2002.
The paragraphs say Mohamed was subjected to "cruel, inhuman and degrading treatment by the United States authorities," including sleep deprivation, shackling and threats resulting in mental stress and suffering.
They conclude that the paragraphs given to the MI5 intelligence service, "made clear to anyone reading them that BM (Mohamed) was being subjected to the treatment that we have described and the effect upon him of that intentional treatment."
British authorities have repeatedly denied complicity in torture.
"The wider point here is that we stand firmly against torture and cruel, inhuman and degrading treatment. We don't condone, collude in or solicit it," Prime Minister Gordon Brown's spokesman Simon Lewis told reporters following the decision.
Ethiopia-born Mohamed was arrested in Pakistan in 2002 and says he was tortured there and in Morocco before being flown to Guantanamo Bay. He was released without charge last year.
The Wednesday decision upholds an earlier High Court ruling ordering officials to make public the secret seven-paragraph summary of U.S. intelligence files. The Foreign Office appealed that ruling, but said Wednesday it would abide by the ruling and posted the paragraphs on its Web site.
Foreign Secretary David Miliband restated the government's backing for the principle that "if a country shares intelligence with another, that country must agree before its intelligence is released." The government had argued that releasing the information would make the U.S. reluctant to share intelligence in the future.
Miliband said he had spoken to U.S. Secretary of State Hillary Clinton about the judgment on Tuesday and they had "reaffirmed the importance of the U.S./U.K. intelligence relationship."
The seven paragraphs, which come from an earlier court ruling, are a judge's summary of a U.S. account of Mohamed's treatment given to British intelligence before he was interviewed by a British MI5 agent in May 2002.
Mohamed's lawyers had long claimed the secret paragraphs prove he was mistreated and that the U.S. and British governments were complicit in his abuse. They have been fighting for access to the documents, along with The Associated Press and other news organizations.
Shami Chakrabarti, director of the rights group Liberty, said a "full and broad" public inquiry into British complicity in torture is needed in light of the information contained in the newly released paragraphs.
"It shows the British authorities knew far more than they let on about Binyam Mohamed and how he was tortured in U.S. custody," she said. "It is clear from these seven paragraphs that our authorities knew very well what was happening to Mr. Mohamed. Our hands are very dirty indeed."
She said it is now evident that British authorities were complicit in the use of torture and benefited from it.
The case began in 2008 when Mohamed was facing a military trial at Guantanamo. His lawyers sued the British government for intelligence documents they said could prove that evidence against him had been gathered under torture.
Mohamed, 31, moved to Britain as a teenager. He was arrested as a terrorist suspect in 2002 in Karachi by Pakistani forces and later transferred to Morocco, Afghanistan and in 2004 to Guantanamo Bay.
He says he was tortured in Pakistan, and that interrogators in Morocco beat him, deprived him of sleep and sliced his genitals with a scalpel.
It isn't clear which country the interrogators were from, but Mohamed has alleged the questions put to him could only have come from British intelligence agents.
MI5 has said it did not know Mohamed was being tortured, or held in Morocco.
Mohamed was charged by the U.S. with plotting with al-Qaida to bomb American apartment buildings, but the charges were later dropped and in February 2009 he was sent back to Britain. That chain of events led to the lawsuit becoming a larger battle for access to information involving the AP, Guardian News and Media, the BBC, The New York Times, The Washington Post and other media organizations.
Mohamed is among seven former Guantanamo detainees suing the British government, accusing the security services of "aiding and abetting" their extraordinary rendition, unlawful imprisonment and torture.
Government officials insist Britain does not condone or participate in torture, but officials have avoided answering specific allegations that Britain participated indirectly by obtaining intelligence from suspects who had been tortured overseas, or sending agents to visit suspects who suffered mistreatment in foreign facilities.
