The real conspiracy in the Catholic church has nothing to do with the Da Vinci Code, says Patrick Wall - it's the cover-up of paedophile priests. Mark Honigsbaum meets the former monk who is leading a crusade to hunt down the perpetrators and bring them to justice

Patrick Wall, or Brother Wall as he used to be known, is feeling a little jetlagged. He has just arrived in London on the red-eye from Los Angeles and has two hours before he has to catch a train to Cardiff - in pursuit of a special sort of criminal.

"I can't tell you much about the perpetrator until I get to Wales and sue his ass," he says, fixing me with a look that could stop the Vatican bells. "Let's just say that there are in excess of 20 victims - boys and girls - and that the alleged abuse dates back to the late 1970s."

Wall is a former Benedictine monk turned international clerical sleuth, and this is his 200th case since joining the LA law firm of Manly, McGuire & Stewart. His job is to hunt down Roman Catholic priests retired by the Vatican in the wake of the sexual abuse scandals that erupted in Boston and other north-American dioceses in 2002. Many of those priests have effectively gone to ground.

Wall's job is to find them, verify their identities, and then serve them with affidavits setting out their alleged involvement in abuse, as the first stage in bringing suits for damages on behalf of their victims in the US courts. It's a tricky job, one that requires a close familiarity with clergymen - and the cunning of Philip Marlowe.

"You want to build up some sort of rapport with the perp [perpetrator] before you serve them, but it can be a little scary because you don't know if they'll have a Smith & Wesson or holy water," he says. "I guess you could call what I do clerical reconnaissance. The point is, I have to be absolutely certain we have the right guy. If we sue the wrong priest, I'm done."

Dressed in a loose-fitting shirt and jacket, Wall, who stands 6ft tall and weighs 18∫ stone, looks like an overfed American tourist. In fact, he's an ex-high school American football star, and before he became involved in trying to stamp out sex abuse in the church, his only ambition was to be a good priest, to teach theology and coach college football. That plan began to unravel in 1992 when, within weeks of taking his vows for the priesthood, Wall was invited to join the church's "sexual abuse response team".

"My job was to firefight cases of sexual abuse - basically, take the place of the perp and calm the waters," he says. "Our definition of success was that no one ever found out about it."

Over the next five years, Wall found himself being shuttled from parish to parish to replace other, similarly failing priests - including one who had abused more than 30 altar boys. "Everywhere I turned, I was running into perpetrators," he says. "In one month alone, we had seven cases against monks go public."

By 1998, Wall had had enough and resigned from the priesthood. Later, he went on to give expert testimony in other sexual-abuse cases. For this, he was accused by his former monastic employers of being "headstrong" - a charge he readily admits - and forging church documents, which he strongly denies.

Now Wall, along with two other former priests turned whistleblowers, has written a book called Sex, Priests and Secret Codes: the Catholic church's 2,000-year paper trail of sexual abuse. The book, he says, contains previously unpublished documents detailing the Vatican's longstanding awareness of the problem of sexual abuse. Its conclusion is that the recent scandals in the US, Ireland and Britain are nothing new, but simply the latest chapter in a story that stretches right back to the founding of the church. And the authors say they have found evidence of deliberate cover-up by the church.

Exhibit A is a hitherto secret document called Crimen Sollicationis - Latin for "the crime of solicitation" - issued in 1962 by the Congregation of the Doctrine of the Faith (CDF), an office attached to the Vatican. Described as a "blueprint for deception and concealment" by lawyers investigating the worldwide sex-abuse scandals, Crimen Sollicationis contains strict instructions for dealing with what the Vatican calls the "worst crimes" - such as allegations of paedophilia and bestiality. Rather than report these offences to the civil authorities, the CDF instructs bishops to investigate them "in the most secretive way" or face the "penalty of excommunication".

Then, in 2001, the CDF followed this document up with a second directive, which ordered bishops to send their reports directly to Rome, where they would be kept safely under lock and key along with other so-called Pontifical Secrets. Wall and his co-authors argue that it is this and not the Da Vinci Code that is the "real conspiracy" at the heart of modern Catholicism. They say it is no coincidence that the person responsible for the promulgation of the second document was none other than the head of the CDF, Cardinal Joseph Ratzinger, now Pope Benedict XVI.

According to Wall and his co-authors, if early church documents are a guide, then the sexual abuse of minors by the clergy has been an "open wound on the body of Christ" for centuries. They say that, as early as AD309, the Council of Elvira, a gathering of bishops and priests from all over the Iberian peninsula who met to discuss theological issues and set canon law, signalled its concerns over paedophilia by ruling that any bishop, priest or deacon caught offending should be denied holy communion even at the time of death. By the eighth century, The Penitential of Bede, a medieval handbook of penances usually ascribed to the Anglo-Saxon theologian and early English historian Bede, had refined the punishments according to rank. Thus laymen caught sodomising children were to be excommunicated and made to fast for three years, while deacons and priests were similarly excommunicated but made to fast for seven and 10 years respectively (bishops were given 12 years of penance).

But the clearest evidence that the early church had the same problem with paedophilia as today comes from the Book of Gomorrah, an 11th-century tract written by St Peter Damian which, while condemning all forms of "immoral" sexual behaviour, holds priests who defile boys in special contempt. Such clerical offenders should not only be publicly flogged but also, writes St Peter, their tonsures should be "shorn" and their faces "foully besmeared with spit". Next, he recommends six months of imprisonment and fasting, followed by a further six months of prayer in solitary confinement. Even after their release, St Peter writes, offenders should never again be allowed to "associate with youths in private conversation or in counselling them".

It was not the only time that church insiders would make such a recommendation - one that contrasts starkly with the more recent practice of transferring priests to neighbouring parishes where they have been free to reabuse. In 1954, Father Gerald Fitzgerald, the founder of the Servants of the Paraclete, an order specialising in the counselling and care of priests with psychiatric problems, recommended that any priest who, as he put it, "tampered with the virtue of the young" should be reduced to lay status. Needless to say, his recommendation to bishops was ignored and, by the late 1960s, the Paracletes were providing psychotherapeutic services to so many priests with paedophiliac tendencies that they coined a shorthand term for the offence: "code 3".

For Wall and his colleagues, the clearest example of the breach of St Peter Damian's injunction to keep priests out of the way of temptation came at the trial in 1998 of Father Oliver O'Grady, a prolific abuser who had been moved from one northern California parish to another in an attempt to avoid scandal. O'Grady eventually admitted abusing 23 children between the ages of four months and 12 years and served seven years in a Californian state penitentiary. Despite repeated instances of sexual abuse, says Wall, the church has been reluctant to take responsibility and root out the problem.

"All the church cares about is keeping the scandal under control, so they can continue to raise money and grow the institution," says Wall. "If they truly had the interests of children at heart, they would never put that priest back into a ministry that could cause him to reoffend."

Meanwhile, the scandals continue to mount: in 2004, 1,092 new allegations were lodged against priests, and in California alone, there are currently 750 cases pending in the courts. Moreover, despite the US Bishops National Review Board's own estimates that there are some 5,000 abusive priests in the US, to date only 150 have been successfully prosecuted - a reflection, in large part, says Wall, of a lack of cooperation from the church. In California, for example, the archdiocese has sought to block the disclosure of confidential counselling records on two priests saying it violates their First Amendment right on religious protection; in other sensitive cases, the Vatican has simply refused to accept the jurisdiction of the US courts.

Although Wall no longer has any faith in Roman Catholic institutions, he and his coauthors do not believe the church is beyond redemption. They dedicate their book to "all the people of God". Today, Wall is happily married to a therapist in California; together, they have a five-year-old daughter, Erin.

"I believe we will be tested by how we treat the widow, the orphan and the alien," says Wall. "In other words, if you have to choose between protecting the institution or protecting children, Christ would have chosen the children."

And with that, Wall, having long since made his choice, rises to his feet and, gripping my hand in his huge fist, announces he has a train to catch. A week later, I email him in Los Angeles to ask whether his mission to Wales was successful. As you would expect from someone whose job it is to keep the Vatican guessing, Wall's reply is somewhat cryptic.

"The first part of the trip was enjoyable and the priest study was not," he replies. I email him back: "So you didn't find the alleged perpetrator?"

"Not yet," he replies, "but I will"