Alan Dershowitz has no truck with false modesty. One of his favourite stories is the one about the pompous rabbi who prostrates himself on Yom Kippur, shouting to God: "I am nothing before you." The equally pompous cantor shouts, even more loudly: "I am less than nothing before you." The lowly shammes, also on his knees, screams: "I too am nothing." To which the rabbi responds: "Look who's claiming to be nothing."

Dershowitz quotes the joke in his recent autobiography, which he called Taking the Stand: My Life in the Law - no doubt because he had already written a book called Chutzpah. And he has every reason for scorning feigned humility.

Now 76 and retired from teaching, he is still proud of becoming Harvard Law School's youngest full professor, aged 28. His photographic memory, inherited from his mother, clearly helped; but, for a boy from Brooklyn who'd been told he was not "college material," it was no small achievement. And yet Dershowitz maintains that his public image - which he describes as "confrontational, unapologetic, brash, tough, argumentative and uncompromising" - is far removed from his private persona: "In my personal life," he writes, "I shy away from confrontation and am something of a pushover."

That was the persona on display at a legal breakfast last month hosted by the Israeli ambassador, Daniel Taub. Dershowitz was in London to collect an award and his comments were off the record. But I am not breaking any confidences by reporting that he gave a well-argued and courteous response to a question from me that might have provoked a sharper retort from someone less sure of his facts.

Listening back to the interview he gave to the BBC's Today programme last weekend, I heard a very different Dershowitz. He was responding to allegations by a 30-year-old woman, named later as Virginia Roberts, that, at the age of 17, she had been forced to have sexual relations with Prince Andrew - allegations that the Duke of York has categorically denied. The claims were made in a lawsuit against Jeffrey Epstein, one of Dershowitz's clients.

That was shocking enough. But what made the story extraordinary, at least to those who knew and respected Dershowitz, was that Roberts had accused him of sexual misconduct, too. And he could not have been more confrontational or uncompromising in denying her claims. "I will not rest or stop until the world understands, not only that I had nothing to do with any of this, but that she, deliberately, with the connivance of her lawyers, made up this story willfully and knowingly," Dershowitz said on Radio 4. Later, he sought permission to intervene in the case, filing a sworn statement in which he denied any sexual or physical contact with Roberts on Epstein's private plane or his Caribbean island.

Dershowitz had become used to the unpopularity that resulted from defending high-profile clients accused of serious crimes, such as OJ Simpson and Claus Von Bulow. "But none of them resulted in false accusations being levelled against me for sexual misconduct based on completely made-up stories," he told a Reuters television reporter.

"This is different. After 50 years, I now know what it feels like to be falsely accused of a serious crime."

In his broadcast interviews, Dershowitz came across as clear, articulate and utterly convincing. There are not many lawyers and still fewer emeritus professors who can speak in pithy soundbites. But then few of them have made a living from popularising the law.

Like all researchers, Dershowitz had built his reputation on academic articles in learned journals. In his mid-30s, though, he was asked by the New York Times to write columns analysing rulings by the US Supreme Court and other legal developments. "I soon discovered that I had a knack for legal writing that was accessible to the general public," he recalls in his autobiography.

Having been chided by his first law teacher for writing the way he spoke, he found he could turn this supposed flaw to his advantage. "I would write for the general public the way I talked to my Brooklyn friends, who were smart as hell but not legally educated." It worked - and he became very well-known.

But something was missing. As an academic, Dershowitz had never practised law. Here in the UK, that would be unexceptionable: apart from those specialising in international law, very few professors are also successful practitioners. But he was different. "I was dissatisfied with being only a teacher and a writer of articles," he recalls. "I wanted to make legal history, rather than just write about others who did." So he built a law practice, starting with unpaid civil liberties work before moving on to free-speech cases. Dershowitz believed practising law made him a better teacher and teaching law made him a more effective practitioner.

In the early 1970s, he agreed to represent a member of the extremist Jewish Defense League on a murder charge. The case coincided with the breakup of Dershowitz's first marriage and his increasing ambivalence towards the strictly Orthodox Judaism in which he had been raised. But he was still determined to become what he regarded as a Jewish lawyer. "I did not fully understand my emotional attachment to a religious culture whose theological underpinning I could not accept, but I decided that a substantial part of my legal career would be devoted to defending Jewish values and Jews in trouble around the world." Carolyn Cohen, his second wife, provided him with the stability and self-confidence that he was missing at that time.

It's many years since I visited Dershowitz at Harvard but I still remember the hate mail he kept pinned up on his office door ("so that students will understand what they face if they become public figures").

These days, he says, much of the abuse he receives is antisemitic, because of his well-known (though far from unqualified) support for Israel. He recalls the JC interview in 2005 with Baroness Deech, who had asked at an Oxford bookshop for a copy of Dershowitz's book The Case for Israel - only to be told by the shop assistant: "there is no case for Israel".

Dershowitz, who once invited President Clinton to join him in shul for Rosh Hashanah, has always been regarded as a liberal-minded Democrat. Because of his advocacy for Israel, he says he is now regarded as a conservative Republican. And yet his support for a two-state solution and his opposition to some Israeli settlement policies has led others to describe him as a "traitor to the Jewish people".

As far as he is concerned, his views have remained constant for 45 years. It's the world about him that has changed, he believes. Anticipating obituaries that he believes will misrepresent him, Dershowitz has written a "letter to the editor", to be sent and published posthumously. "I supported Israel," he writes, "not despite my liberalism but because of it - and because I have always defended just causes against unjust attacks."

Dershowitz has now told a court that those just causes include himself. I wonder if Roberts or her lawyers understood just whom they were taking on.

Joshua Rozenberg trained as a lawyer before becoming a legal journalist and commentator.