When the impressionist Rory Bremner was researching a sketch about lawyers who give clients the advice they want to hear regardless of what the law actually says, he phoned Philippe Sands QC.

This is territory that Sands, professor of law at University College London, has made his own. In Lawless World (Allen Lane, 2005), he explored how fundamentals of international law, established largely by the Americans and the British, were being undermined by George W. Bush and Tony Blair. He also offered the first full account of how Lord Goldsmith, the Attorney-General, changed his advice on the legality of the war in Iraq.

In 2007, Sands for the prosecution and Julian Knowles for the defence interviewed a number of distinguished witnesses to test the grounds for indicting Blair for the crime of aggression against Iraq. The transcripts were edited into a play, Called to Account, by Richard Norton-Taylor, and audiences got a chance to vote at the end. Both Sands and Knowles are members of Matrix Chambers in London. So, intriguingly, is Blair's wife Cherie Booth.

If Lawless World provides the big picture, Sands's gripping new book, Torture Team, is startlingly miniaturist. He set out to dissect "the process of decision-making in relation to a single piece of paper, a memorandum signed by Donald Rumsfeld on 2 December 2002". This took him on a journey round America - and into an unexpected confrontation with the traumatic history of his own family.

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Sands sees himself primarily as an academic, but it is a role with two significant sidelines. He has a permanent though nominally half-time post at UCL, where he has an office, teaches courses, supervises doctoral students and runs the Centre on International Courts and Tribunals. He also takes three or four cases a year as a barrister at Matrix, which he helped set up in 2000.

He was lead counsel for Human Rights Watch in the extradition proceedings against former Chilean dictator Augusto Pinochet in 1998, and he has also been involved in, among other things, the investigations into alleged corruption at British Aerospace, a boundary dispute between Honduras and Nicaragua and a claim by Croatia against Serbia relating to the massacre at Vukovar. Interestingly, Sands says, there is a "virtual monopoly of academics", and notably British ones, pleading at the International Court of Justice. "It's a historical curiosity," he explains. "Large corporate firms don't get that sort of work. For some reason, governments are more comfortable with academics."

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The "action memo" that Rumsfeld signed in December 2002 - when he was US Defence Secretary - made "legally available" a number of new and more aggressive "counter-resistance techniques". When it was released at a press conference in June 2004, the Bush Administration had its story in place: the decision had been taken in response to requests from the commanding officer in Guantanamo, Major-General Michael Dunlavey, backed by his legal adviser Lieutenant-Colonel Diane Beaver. (Dunlavey had no warning of the press conference - he woke to find the headline "Dunlavey accused of torture" on the front page of his local newspaper.)

Like many others, Sands thought there was something fishy about the official scenario. The safeguards built into the Geneva Conventions, as well as "the US military's long-established constraints on cruelty and torture, dating back to President Lincoln in 1863", had been abandoned. Many of the main players in the process were "civilian lawyers who were political appointees and close to the President, not military lawyers". All his instincts told him that the changes had been imposed from the top.

What spurred Sands into writing, however, was a day's sick leave when he happened to watch the film Judgment at Nuremberg (1961), a fictionalised version of the so-called Justice Cases brought in 1947 against several lawyers active in the Nazi period. While the scale of the crime was immeasurably different ("I'm not alleging that there was mass torture at Guantanamo. My view is that there wasn't; it was extremely limited"), he determined to unearth the process that led to the crucial memorandum drafted by Rumsfeld's general counsel Jim Haynes - and to throw light on "how lawyers allow abuse to occur".

Rather to Sands's surprise, all the major figures agreed to see him - right up to the chairman of the Joint Chiefs of Staff, the commander of Southern Command (responsible for Central and South America), the Undersecretary of Defence for Policy and even Haynes himself. Sands can only speculate as to why he was granted such unprecedented access. "I think they spoke to me because I was not American; I was not a journalist; I was a lawyer, a barrister and a QC - those three factors combined to give them a sense that I would treat them fairly, I would put the case for and against. There is a sort of code of honour among lawyers that you will treat each other respectfully."

Ever since he was training as a barrister and doing things such as "appearing before Hackney magistrates when someone was caught stealing sausages", Sands has been well aware that legal judgments are not only about "the mechanical application of fact".

You soon learn, he says, that "law is not just something defined by judges but has shadings, colourings, can be bent in different directions, and has a connection with political reality. That's the same at Hackney magistrates or the World Court." Furthermore, of course, individual lawyers, caught up in personal and political power structures, are under pressures that influence their behaviour.

As Sands interviewed key characters, dimensions emerged that he could never have guessed from the published record. Beaver, the lawyer at Guantanamo, for example, "appears in the first book, when I'd not met her. She wrote a legal opinion that I think is ghastly, and I'm extremely rude about it in Lawless World. I wanted to get to know her, find out who she was, try to understand her, and that required approaching her with a completely open mind. I still think her opinion is ghastly and told her so, but I now understand the context in which she found herself and have completely revised my view of her."

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What helps make Torture Team so powerful is that it largely explores the crucial issues - what the military thought of the new guidelines, whether the techniques used at Guantanamo count as torture, whether Administration lawyers could face criminal charges, once out of office, if they visit certain places - through interviews with individuals. Since the only legal precedents for prosecuting lawyers for complicity are the German Justice Cases (known as United States v Josef Altstotter and Others), which formed the basis for Judgment at Nuremberg, Sands also travelled to Germany to study these.

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Altstotter, Germany's chief of the Civil Law and Procedure Division under the Nazis, was found "guilty of membership of a criminal organisation, the SS, with knowledge of its illegal activities". His son Ludwig is utterly convinced that this was a miscarriage of justice and that his father had no idea what was going on. The most damning document on which his name appears in effect denies the right of appeal to Jews who had been deported from Vienna to the camp at Theresienstadt. By a strange coincidence, Sands's maternal grandparents were part of that very transport. Although he was unconvinced by Ludwig's arguments, Sands was disconcerted to find him a very likeable man.

Sands considers the question of whether torture works in delivering reliable intelligence and concludes that - except perhaps in very specific circumstances that did not apply in Guantanamo - it does not. As he travelled, however, he kept hearing about two opponents to this view. One was Alan Dershowitz, the high-profile lawyer and legal scholar at Harvard University who wrote a book called Why Terrorism Works (2003). The other was someone called Jack Bauer - who turned out to be a fictional character for whom "rules don't apply", the hero of the TV series 24.

Torture has always been horribly common, so why does Sands see such significance in a legal decision that may have directly affected only a single detainee? "These things happen. But there's a very big difference between things happening on the battlefield, in the heat of the moment and without formal authorisation, and a situation where the highest levels of government in the US authorise it. It opens the door to something qualitatively different.

"In physical terms, of course, it's not worse than the mass torture that is happening today in many countries of the world. The reason that it's so important is that the US has always held itself out as doing things differently - and has done things differently. It has been a leader in developing international human rights law and international humanitarian law. If a country like the US opens the door to this type of behaviour in a formal sense, the world has changed. It's vitally significant.

"I work as a barrister for a large number of governments. I have been told by foreign ministers, I've even been told by a president, that - on the basis of the legal advice and the documents that led to the decision of December 2002 - they now see no reason why they can't do the same thing. It has opened the door to the legitimation of those types of action in foreign countries. And worse, it has made it impossible for the US to say to those countries you can't do them. So moral authority has gone, and there's no longer any difference between us and them. We have seen the price Britain and America have paid for that over the past five years. It's a big price, and it's going to take a generation to get over it."

This is a lesson, Sands adds, that we should have learnt from history. "The Brits used five techniques like this in the early Seventies in Northern Ireland against IRA prisoners, and on some accounts this extended the life of that conflict by 15 or 20 years. It so enraged certain sections of the community that it made it impossible to address the issues."

Anyone opposed to torture is always challenged by the "ticking bomb" scenario (is it justifiable to torture a person who has placed a bomb in a school in the hope of gaining information that could save lives?). One can debate whether it is just a nice dilemma for a seminar in moral philosophy or whether it can ever plausibly apply in the real world. Nonetheless, I decided to put it to Sands.

"Could I justify torture in circumstances where a bomb was planted in a school and my children were going to be affected?" he replies. "The easy answer is to say it's hypothetical and I'm not going to go there. But that's a cop-out. I think one has to say that 'never' means 'never'. Once you open the door, it cannot be shut. The threshold becomes very difficult to define. Once you accept that there are some circumstances where the deliberate use of force is justified, it is inevitable that the door is open and it will be used again in other circumstances. That door has to be kept shut. I think that's where I come out ... ".

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