Their daily penance is also physical.

It involves a sharply barbed chain of metal spikes, which numeraries wrap around their thighs, as tightly as they can bear. The strap is called a cilice.

They wear it for two hours, every day. It leaves small, bloody holes in the skin. These physical acts are called 'corporal mortification'.

And so the stories go.

When a knock comes on their door in the morning, numeraries are encouraged to jump out of bed and kiss the floor, saying "Serviam" (Latin for "I will serve"). They call this 'the heroic minute'.

Women sleep on wooden boards, laid on top of their mattresses. Men occasionally sleep on the floor. Both sexes sleep without a pillow once a week. Every day they have cold showers as penance.

They give their entire salaries to Opus Dei . . . excluding personal expenses . . . and may not hold their own bank accounts. Their incoming and outgoing personal mail is read by 'spiritual directors'. Contact with families is extremely limited, even discouraged.

And so the stories go.

Opus Dei was founded less than 80 years ago by Josemaria Escriva, but already its status as the most controversial branch of the Catholic church is absolutely unrivalled. In 1982, Pope John Paul II established Opus Dei as the first ever prelature of the Catholic church, meaning its members are governed by their own bishop. There was further controversy when Escriva was put on the fast track to sainthood, by-passing the less favoured Pope John XXIII.

Over the last 30 years, numerous accusations have been levelled against the organisation, all of which seem too bizarre to be true. And yet the stories persist. And they all say the same thing.

"They are controlling people's lives and causing serious psychological harm, " says Dianne DiNicola, executive director of the Opus Dei Awareness Network (Odan), which was established in 1991 to alert people to some of Opus Dei's practices. "They really damage people, and the effects don't just go away. A man contacted me recently who had left them 20 years ago, and he said he was still dealing with psychological problems as a result."

DiNicola was one of the founding members of Odan. Her zeal stems from nearly losing all contact with her daughter Tammy, who was recruited into Opus Dei during her freshman year at Boston College.

"Tammy was 18, it was her first time away from home and she got a part-time job at the college diner, " says DiNicola. "One of the students working with her was from Opus Dei, and Tammy got on her radar. She befriended Tammy, and invited her to a retreat. She didn't tell her she was from Opus Dei, or that it was an Opus Dei retreat. They don't divulge information about themselves, if they can help it."

Tammy was from a practising Catholic family. Her part-time job suggested a good work ethic. She was young and idealistic. All of these things, says DiNicola, made her an obvious target for Opus Dei.

"They work on a person, prepare them, " she says. "They wait until they seem ready, and then they tell them they have a vocation. In Tammy's case, the recruiter was very determined to get her to join. Tammy didn't tell us anything. Suddenly we were seeing this nervous girl who just didn't seem to be there any more, and we didn't know what was happening."

Initially, Tammy agreed to join Opus Dei as a supernumerary, meaning she could get married and live at home. However, she was convinced instead to commit to the life of a numerary . . . which involves being celibate, living in an Opus Dei centre, and partaking in corporal mortification.

Tammy wore a cilice every day, and whipped herself every week.

"She lived with them for three years, and we saw less and less of her, " says DiNicola. "She had to ask permission to come and go from the centre, and she wasn't allowed to come to her sister's wedding, or to visit us for the holidays. All the letters we wrote her were read first by her spiritual director. She had to ask permission before reading a book. She had to ask permission before putting on music. She lost the ability to think because she wasn't making any decisions for herself. It's frightening how they slowly get complete control over people."

The DiNicolas tried talking to an Opus Dei priest about their concerns, but left the meeting feeling wrong for questioning the organisation.

"They made us feel like we were bad people for protesting, " she says. "And we started thinking maybe they were right. I mean, if the pope had given them prelature, they had to be okay.

But we couldn't accept that for long."

Eventually, the DiNicolas staged a family intervention. They managed to get Tammy to come home for her graduation party, and for 24 hours, her family, extended family and friends tried to communicate with her. When it came time for her to leave, she said she wasn't going back.

"It was a miracle that it worked, " says DiNicola. "I believe it was the grace of God that we managed to show her what was happening."

Now, Dianne DiNicola devotes her life to helping other people who have had negative experiences with Opus Dei. She says Dan Brown's The Da Vinci Code . . . the movie version of which opens worldwide later this week . . . has helped this quest by forcing Opus Dei into the open. "When it comes to describing the blind obedience of members of Opus Dei, the book does a brilliant job, " she says.

"But it is sensationalist [the main Opus Dei character, Silas, is a murderous monk] and because it's sensationalist, it allows Opus Dei to say 'Look at this, this is a ridiculous representation.'" Indeed, this is exactly how 35-yearold Dony MacManus, an associate numerary from Dublin, regards Brown's multi-million-selling tome.

"To me, The Da Vinci Code is as nonsensical as Scooby Doo, " he says.

"The idea of the evil-looking monk going around whipping himself is just ridiculous and I could laugh at it except that there are people out there taking it seriously. Opus Dei is my family and for someone like Dan Brown to say those things is quite hurtful, never mind libellous."

Michael Kirke, a numerary from the Irish Opus Dei information office, also has little time for Opus Dei's representation in Brown's novel. "The way the monk uses the cilice is gross, that's the only word for it, " he says. (Silas wears the cilice for extended periods of time, and whips himself ferociously when he fails his first task. ) However, Kirke admits that many Opus Dei numeraries do use the cilice every day.

"It's a very simple little thing, just a little bit of penance, " he says. "You're sitting at your desk and you know it's there, and you'd prefer not to have it on. But Jesus Christ had a crown of thorns put on his head and this is very small by comparison."

Kirke also confirmed the ritual of self-whipping once a week, although he refers to it as "discipline". "It sounds terrible to put it like that, to say people whip themselves, " he says.

"Discipline is done in the same kind of spirit as people who go to Lough Derg and fast for three days."

The 'heroic minute' of kissing the floor first thing in the morning is also a true story. "We would find it strange that people would make anything out of that, " he says. "It's a little graphic thing to help people start their day."

Wooden boards on the mattresses, no pillows, cold showers, all of these have basis in fact, but Kirke insists that they are all voluntary activities.

It is also a personal decision to censor reading material, he says. "For example, if a friend told me about a really good book, but said there was erotic material in it, then I wouldn't want to read it because it would be morally harmful, " he said. "There is an index of books that used to be forbidden by the Catholic church, and I would think that most numeraries would avoid anything on that list. If I am in doubt about whether to read something, I seek advice from my spiritual director."

It is only when speaking about Dianne and Tammy DiNicola, and the work of Odan, that Kirke's demeanour strays from absolute calm. "Clearly, Tammy felt a lot of things, and maybe it was a mistake for the people who brought her into the vocation to do so, " he says, his voice rising, his words coming more quickly. "But maybe people need to take full personal responsibility for their own actions. No-one has ever asked me to do anything in Opus Dei that I haven't wanted to do. The suggestion that we are denied freedom doesn't make any sense to me, either from my personal experience or from the experiences of the people I have helped to find vocations."

The conversation almost ends. But Kirke wants one more issue cleared up. "About the secrecy, " he says. "It's a contradiction of everything we stand for to be secretive. We are encouraged to shout from the rooftops about our faith. We want everyone to know about The Work. Ultimately, we just want to spread The Word."

Da Vinci decoded: the book in 100 words Mary Magdalene was married to Jesus and pregnant with his child, and women are actually far more important than men. These are the secrets that the Catholic Church is determined to protect. Using Silas, a murderous albino monk, the Church and Opus Dei set about killing anyone who threatens to reveal the truth. Their main targets are a Harvard University professor who is afraid of elevators, and a cryptologist who turns out to be the long lost greatgreat-great (etc) grand daughter of Jesus. And the Holy Grail is actually hidden in the Louvre, where the story all begins.