Former US President George W Bush has said British lives were saved by the use of information obtained from terrorist suspects by "waterboarding".
The Bush administration classified it as an "enhanced interrogation technique", but waterboarding is widely regarded as torture and in 2009 President Barack Obama banned its use.
It is believed to have been used on three al-Qaeda prisoners under the Bush administration.
What is waterboarding?
Waterboarding involves a prisoner being restrained on his back with his feet at a level higher than his head, or hung upside down. A cloth is placed over the prisoner's face or pushed into his mouth. Sometimes a plastic film is used. Water is then poured on to his face and into his nose and mouth. The prisoner gags almost immediately as the water starts entering the lungs. As he starts to feel he is drowning, he typically panics and struggles, and his body goes into spasm. Waterboarding can result in brain damage, broken bones and psychological damage.
Does it come under a technical definition of torture?
Human rights groups and many governments say that it does. The United States government under George W Bush did not agree. But President Obama thinks it is torture and so does his CIA chief Leon Panetta.
Torture is defined by the 1949 UN Convention against Torture as "any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person..." in order to get information.
The US is signed up to the Convention. The eighth amendment to the US Constitution banning "cruel and unusual punishment" is also held to prohibit torture.
The US legal code defines torture as an action "specifically intended to inflict severe physical or mental pain or suffering".
So why did the US use waterboarding?
Because it did not at the time classify waterboarding as torture and regarded it as an effective method in a small number of cases.
It made a distinction between "torture", which it accepted is banned by US and international law, and so-called "enhanced interrogation techniques".
These techniques included not only waterboarding but sleep deprivation, subjection to cold and long periods of standing, and some slapping.
Has President Obama banned waterboarding by all US government agencies, including the CIA?
Yes. He has brought the CIA into line with the US military, which in 2006, in a new army manual on collecting intelligence, banned torture and degrading treatment, including waterboarding, forced nakedness, hooding and sexual humiliation.
The manual's publication followed the scandals at Abu Ghraib prison in Iraq and the passing of the Detainee Treatment Act in 2005, which prohibited the "cruel, inhuman, or degrading treatment or punishment" of detainees.
President Bush excluded the CIA from the restrictions imposed on the military. He did so in an executive order in July 2007, which sought to define the American commitment to the Geneva Conventions' Common Article 3 prohibition on cruel, humiliating and degrading treatment and torture.
The order declared that a CIA "programme of detention and interrogation" complied with the Geneva Conventions.
The order listed interrogation methods and practices that are not allowed. These ranged all the way from murder and rape to acts of humiliation.
The banned methods did not, however, include the so-called enhanced interrogation techniques.
Is waterboarding effective?
According to ex-CIA officer John Kiriakou, al-Qaeda suspect Abu Zubaydah "broke" within half a minute. Abu Zubaydah said later that he had made things up to satisfy his interrogators.
However, the New York Times reported on 19 April 2009 that a Justice Department memo revealed that CIA interrogators used the waterboarding technique 183 times on Khalid Sheikh Mohammed, the admitted planner of the September 11 attacks, and 83 times on Abu Zubaydah.
The figures undermine the suggestion that the technique produces rapid results.
The arguments about the efficacy of waterboarding reflect all arguments about similar methods. Do they produce information or lies? Can the information be obtained by other means? And are they counter-productive?
How was waterboarding justified legally?
In April 2009, President Obama released a series of legal memos written by lawyers under the Bush administration that sought to justify the use of waterboarding and other methods. One memo, from 2002, contained legal authorisation for a list of specific harsh interrogation techniques, including pushing detainees against a wall, facial slaps, cramped confinement, stress positions and sleep deprivation. The memo also authorised the use of waterboarding and the placing of a detainee into a confined space with an insect.
The memos argued that the methods were not "cruel, inhuman or degrading" under international law.
One suggested the practice "would not violate" any US statute, the constitution or any treaty obligation.
Were any safeguards put in place?
The memos show that in 2004 a CIA request to use waterboarding was approved if certain conditions were observed. These included ensuring that the practice was conducted under the supervision of a physician or psychologist with the authority to stop it.
Another memo said waterboarding would only be used when the CIA has "credible intelligence that a terrorist attack is imminent" and there were "credible indicators that the subject has actionable intelligence".
Will anyone be prosecuted for torture?
President Obama says that those who conducted the interrogations will not be prosecuted. He stated: "Those who carried out their duties relying in good faith upon legal advice from the Department of Justice... will not be subject to prosecution." Initially the White House also indicated that those who authorised the techniques would also not face prosecution but after protests from human rights groups and some members of Congress, the president said later that the attorney general would investigate and he would not prejudge the outcome. He also left the door open to a special commission to examine the Bush administration's use of interrogation techniques.
Has there been any defence of waterboarding?
Vice-president Cheney said after the release of the memos that other memos showing the "success" of the interrogations using waterboarding and other methods should also be made public. He said he wanted to give the American people "a chance to see what we obtained and what we learned and how good the intelligence was."