When the US military trains soldiers to resist interrogation, it uses a torture technique from the Middle Ages, known as "waterboarding". Its use on terror suspects in secret US prisons around the world has come to symbolise the Bush administration's no-nonsense enthusiasm for the harshest questioning techniques.

Although waterboarding has been considered torture for over a century and the US military is banned from using it, controversy over its continuing use by the CIA may be about to derail the appointment of President Bush's candidate for US Attorney-General.

Michael Mukasey, a retired federal judge from New York and a veteran of several al-Qa'ida trials, was questioned by a Senate committee on Tuesday and refused to say whether waterboarding was illegal.

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Instead, he called the technique "repugnant to me" and promised to investigate further if he was confirmed in the job. He explained that he could not say yet whether the practice was illegal because he had not been briefed on the secret methods of US interrogators and he did not want to put the CIA officers who used it in "personal legal jeopardy".

Even though Congress banned waterboarding in the US military in 2005, it did not do so for the CIA. As a result, Mr Mukasey told senators, it was uncertain whether this technique or other harsh methods constituted "cruel, inhuman or degrading" treatment. His answers did not satisfy the Democrats, however, and his approval now hinges on whether he is willing to say the torture method is against US law.

In a further embarrassment for Mr Bush yesterday, Malcolm Nance, an advisor on terrorism to the US departments of Homeland Security, Special Operations and Intelligence, publicly denounced the practice. He revealed that waterboarding is used in training at the US Navy's Survival, Evasion, Resistance and Escape School in San Diego, and claimed to have witnessed and supervised "hundreds" of waterboarding exercises. Although these last only a few minutes and take place under medical supervision, he concluded that "waterboarding is a torture technique – period".

The practice involves strapping the person being interrogated on to a board as pints of water are forced into his lungs through a cloth covering his face while the victim's mouth is forced open. Its effect, according to Mr Nance, is a process of slow-motion suffocation.

Typically, a victim goes into hysterics on the board as water fills his lungs. "How much the victim is to drown," Mr Nance wrote in an article for the Small Wars Journal, "depends on the desired result and the obstinacy of the subject.

"A team doctor watches the quantity of water that is ingested and for the physiological signs which show when the drowning effect goes from painful psychological experience to horrific, suffocating punishment, to the final death spiral. For the uninitiated, it is horrifying to watch."

The CIA director Michael Hayden has tried to defuse the controversy. He claims that, since 2002, aggressive interrogation methods in which a prisoner believes he is about to die have been used on only about 30 of the 100 al-Qai'da suspects being held by the US. Meanwhile, a CIA official told The New York Times waterboarding had only been used three times. The Bush administration has suggested that the interrogation of al-Qai'da's second-in-command, Khalid Sheikh Mohammed, was a success thanks to the technique, and used this to justify continued aggressive interrogations of suspects in secret CIA prisons.

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While US media reports typically state that waterboarding involves "simulated drowning", Mr Nance explained that "since the lungs are actually filling with water", there is nothing simulated about it. "Waterboarding," he said, "is slow-motion suffocation with enough time to contemplate the inevitability of blackout and expiration. When done right, it is controlled death."

Mr Nance said US troops were trained to withstand waterboarding, watched by a doctor, a psychologist, an interrogator and a backup team. "When performed with even moderate intensity over an extended time on an unsuspecting prisoner – it is torture, without doubt," he added. "Most people cannot stand to watch a high-intensity, kinetic interrogation. One has to overcome basic human decency to endure watching or causing the effects. The brutality would force you into a personal moral dilemma between humanity and hatred. It would leave you to question the meaning of what it is to be an American."

Mr Mukasey's nomination goes before the Senate next week. Three Democratic presidential candidates, including Hillary Clinton, have already said they will not support him. However, the White House said yesterday that it did not believe his nomination was in jeopardy.

'I felt I was drowning and I was in terrible agony'

Henri Alleg, a journalist, was tortured in 1957 by French forces in Algeria. He described the ordeal of water torture in his book The Question. Soldiers strapped him over a plank, wrapped his head in cloth and positioned it beneath a running tap. He recalled: "The rag was soaked rapidly. Water flowed everywhere: in my mouth, in my nose, all over my face. But for a while I could still breathe in some small gulps of air. I tried, by contracting my throat, to take in as little water as possible and to resist suffocation by keeping air in my lungs for as long as I could. But I couldn't hold on for more than a few moments. I had the impression of drowning, and a terrible agony, that of death itself, took possession of me. In spite of myself, all the muscles of my body struggled uselessly to save me from suffocation. In spite of myself, the fingers of both my hands shook uncontrollably. 'That's it! He's going to talk,' said a voice.

The water stopped running and they took away the rag. I was able to breathe. In the gloom, I saw the lieutenants and the captain, who, with a cigarette between his lips, was hitting my stomach with his fist to make me throw out the water I had swallowed."

From: Alleg, Henri, The Question, University of Nebraska Press, Lincoln: 2006; original French edition © 1958 by Editions de Minuit