Ilich Ramirez Sanchez - alias Carlos the Jackal - in court with Isabelle Coutant-Peyre in November 2000. Picture: AFP

Isabelle Coutant-Peyre is putting the case for the defence: to explain how she, a respected French lawyer raised in a bourgeois family and educated by nuns, fell in love with and married a Marxist-Leninist revolutionary turned international terrorist.

This being France, however, the conversation turns inevitably to sex. Or rather the lack of it, since her Venezuelan husband has spent every day (and night) of their married life languishing in a high-security jail.

"Everyone always asks about that aspect of our marriage," she says. "They want to know how you can marry someone and not be able to you know. I've come up with the perfect response. I say I've an ideal husband because he leaves me alone all night."

She laughs throatily and lights another Cuban cigarillo. We are sitting in the Palais de Justice bar in Paris, where she swaps her lawyer's robe for a chic leather jacket. Gamine, with jetblack hair, kohl-rimmed eyes and a husky voice, she has an air of rebelliousness. Her mobile phone, which has already rung several times, trills again. She lowers her head and mutters into it: "I'll tell you later. Me too, me too." Snapping the phone shut, she announces: "That was Carlos."

There is no need to ask "Carlos who?" Ilich Ramirez Sanchez ? alias "Carlos the Jackal"? was once the world's most wanted terrorist before anyone had ever heard of Osama bin Laden. He has been implicated in a number of international terrorist attacks and recently, in an interview on French television, claimed responsibility for killing more than 1500 people in the cause of Palestinian liberation (he is a convert to Islam). He was imprisoned in 1997 for the murder of two French policemen and an informant 22 years earlier.

Today he is Madame Coutant-Peyre's husband and the author of excruciating love poems penned from his cell. "I am jealous of the sun that tans you," he writes. "Of the shade that caresses you; Of your sheets that do not cover me. Of your legs not intertwined with mine."

Given the national outrage that her marriage in August 2001 caused, Coutant-Peyre might have thought it wise to keep her head well below the parapet and the lovelorn verse for private consumption. Instead she is causing another furore with her recently published autobiography Epouser Carlos:Un Amour sous Haute Tension (Marrying Carlos: A High-Pressure Love) detailing their love affair, and including the poems.

The book is intended as an explanation what possessed this professional, attractive woman to marry him? Yet its 279 pages, many of them filled with legal arguments (she believes Carlos was abducted from the Sudan and flown to France illegally), leave the question unanswered. "Some people view the marriage as a mistake, some a fault, a provocation, a manipulation, a folie," Coutant-Peyre admits. "A lot of my friends thought I was mad. But they know I'm not mad, so they realised it must be something else."

Exactly what else? Friends do not doubt that, at 50, Coutant-Peyre is passionately in love with Carlos, who is four years older. They concede the couple may be "on the same cultural and intellectual wavelength", as one told me, but they remain baffled by the marriage. Even the writer Joseph Vebret, who worked for two years with Coutant-Peyre on her autobiography, told me he was surprised.

"The marriage was symbolic," Coutant-Peyre says. "We wanted to declare that we loved each other for the rest of our lives. The fact is that we love each other like millions of others in the world. It's as simple as that."

Yet it is not all that simple. The couple met in 1997 when Coutant-Peyre was working with the lawyer Jacques Verges, nicknamed the Devil's Advocate, whose clients have numbered Klaus Barbie and Slobodan Milosevic, and may soon include Saddam Hussein. Verges had established a team to defend Carlos, Public Enemy Number One at the time and the man behind the seizure of 70 hostages at the Opec oil ministers' meeting in Vienna in 1975 and the 1972 Munich Olympics massacre. He asked Verges to include Coutant-Peyre (who had earlier represented his first wife, the German revolutionary Magdalena Kopp) in his team.

Coutant-Peyre says that it was "an honour I couldn't refuse". In her book, she describes in breathless terms her first meeting with Carlos in Le Sante prison in Paris. It was, she writes, a coup de foudre, a bolt of lightning. "He took my hand and kissed it in the most courteous way. At that moment a wave of recognition passed between us."

Shortly afterwards Carlos sacked Verges and made Coutant-Peyre head of his defence team. Although he was jailed for life on Christmas Eve the same year, she continued (and continues) to defend him in various legal and appeal actions. For more than two years, she visited him in prison, during which time they "kept their distance". It was only in 2000 that "the barriers between us exploded and the intensity of our feelings for each other was revealed".

They married a year later in a Muslim ceremony in a glass-walled prison room containing a few sticks of furniture covered in graffiti. There was a solitary witness and they were watched over by guards. Carlos recited verses from the Koran, they signed a sheet of paper, exchanged rings (he gave her a platinum one from Cartier) and kissed. Then he went back to his cell and she returned to her apartment.

Coutant-Peyre does not pretend it was legal, in civil terms. She had been separated from her husband, a public servant, for four years when she met Carlos, but they were not divorced. She is now, but Carlos is still married to his Palestinian second wife, Lana Jarrar, though she has disappeared and cannot be traced. "It's recognised as a marriage by Muslims the world over," says Coutant-Peyre. "And we'll have a civil ceremony one day when Carlos is free."

Aware that the ceremony would make headlines, she immediately told her three sons Florent, 25, Gabriel, 24, and Aurelien, 12, who were "just fine" about it although they didn't see why he was getting married.

Her parents were horrified. They separated when she was nine, and Isabelle, their only daughter, was brought up by her father, a distinguished businessman. She was educated at a Catholic boarding school, followed by the University of Science and Politics, before studying law. Coutant-Peyre admits she had always been rebellious "perhaps because I grew up surrounded by men, which made me tougher" and is famed for her confrontational approach in court.

It is hard to understand what Coutant-Peyre gets out of this liaison. The French authorities have accused her of being deliberately "provocative" and threatened to have her disbarred. She simply shrugs that she would have more time for her hobbies (painting and playing the piano) if she did not work. Indeed, she seems almost to relish the prospect of a battle with the authorities.

"I'm waiting for someone to show me the law stipulating that a lawyer does not have the right to marry a client, even if he's a prisoner."

Coutant-Peyre believes that the authorities acted out of spite when they moved Carlos to a high-security unit 260 kilometres from the capital after their marriage. "It's full of psychopaths, murderers and sexual criminals," she says, without a trace of irony.

Coutant-Peyre has always courted controversy. She has become a notorious champion of lost (or at least highly dubious) causes, claiming to take "great pleasure in defending political prisoners, freedom fighters or prisoners of conscience" sometimes for no fee. She admits being vehemently anti-American, describing the US as "imperialist" and "tyrannical".

Today, Coutant-Peyre is in the middle of a controversy, defending a group of Breton separatists who allegedly bombed a McDonald's restaurant, killing a woman. "It's true," she shrugs, "I don't make life easy for myself. I don't have a life which is tranquil and calm and I do choose to defend some ? I wouldn't call them lost causes, let's just say some difficult cases."

She is relentless in her defence of Carlos, whom she explains, does not believe the terms "terrorist" or "terrorism" are pejorative when they describe acts committed to avoid something worse happening, or more people being killed. He believes terrorism is unofficial action that influences the politics of a state. "He is not a criminal. He is a political man, a freedom fighter, a revolutionary. And he has been very badly treated."

Her opinion would cause profound offence to the 1500-plus families of her paramour's victims. Perhaps this is the defence lawyer doing her job, but it appears to be the devoted wife talking. In any case, she clearly believes every word of it.

- The Sunday Telegraph