Of all the matters lurking in the overflowing in-tray of the new Pope Benedict XVI, the long-running and emotive issue of paedophile priests is the most damaging - not just to the church, but to his own personal standing. The new pontiff has been accused of failing to investigate a series of abuse claims made against one of his predecessor's closest supporters - a failure which has come to be seen as typical of the Catholic Church's determination to keep a lid on the scandal of priests who breach their position of trust.
The story goes back to the Nineties when the new Pope - then Cardinal Joseph Ratzinger - was head of the Congregation for the Doctrine of the Faith, the Vatican office responsible for investigating abuse claims.
One of the most high-profile of such claims was made by Professor José Barba Martin, a 68-year-old professor of humanities at the Instituto Tecnológico Autónomo de México. He is one of nine former members of the Rome-based Legion of Christ who allege they were abused by the religious order's powerful founder, Marcial Maciel.
Maciel, 84, set up the ultra-conservative order in 1941. Today it has around 500 priests and 2,500 seminarians in 20 countries. It may be small, but its influence is significant. Maciel, who stood down as the order's head in January because of his age, was a confidant of Pope John Paul II, who praised him as an 'efficacious guide to youth'. For decades, Barba Martin kept silent about the abuse claims which are strenuously denied by Maciel and the Legion of Christ. It says the accusers are 'attempting to tar the Vatican... with the stain of these false allegations.'
But, in December 1994, Barba Martin saw an advertisement in a newspaper in Mexico City celebrating Maciel's half a century as a priest and picturing him with John Paul II, and decided to act. At first Barba Martin and other former members of the Legion attempted to raise their allegations with the Vatican, their case becoming a cause célèbre among groups trying to expose abuse within the church, and what they saw as the efforts of its senior hierarchy to cover it up.
'We are modest instruments of history. We have to play our own part to produce a possible change,' Barba Martin told The Observer. 'The problem of sexual abuse within the church has been "cloned" to second and third generations. It has become an epidemic situation.'
The nine filed their case before the Vatican's courts, and persuaded intermediaries to carry a letter to Ratzinger outlining abuse allegations against Maciel. According to one of the intermediaries, Father Alberto Athie, when confronted with the allegations Ratzinger simply said Maciel had brought many 'benefits' to the church and that it was a 'touchy problem'. The Vatican denies Athie's claim.
On 24 December 1999, Ratzinger's secretary, Father Gianfranco Girotti, wrote to the men saying that their claims - many allegedly corroborated by each other's detailed testimonies - had been examined but, for the time being, the Vatican considered the matter closed. In a last-ditch attempt to persuade Ratzinger to change his mind, another letter was despatched to him in 2002 through an intermediary. It went unanswered.
For support groups campaigning on behalf of abuse victims, Ratzinger's apparent reluctance to investigate the claims against Maciel was a crushing blow, but one that did not surprise critics of the new Pope.
Ratzinger's role in protecting the church against scandal became apparent four years ago. In May 2001, he sent a confidential letter to every bishop in the Catholic church reminding them of the strict penalties facing those who referred allegations of sexual abuse against priests to outside authorities.
The letter referred to a confidential Vatican document drawn up in 1962 instructing bishops on how to deal with allegations of sexual abuse between a priest and a child arising out of a confessional.
It urged them to investigate such allegations 'in the most secretive way... restrained by a perpetual silence... and everyone... is to observe the strictest secret which is commonly regarded as a secret of the Holy Office... under the penalty of excommunication'.
'What really bothers me about this document is the way it suggests that what happens in the confessional should stay in the confessional,' said Carmen Durso, a Boston lawyer who has represented scores of American victims abused by priests.
'In the cases I've dealt with, the paedophiles frequently use the confessional to try and initiate contact with youngsters.' Ratzinger also oversaw the creation of Essential Norms, a 2002 document that reaffirmed the church's right to retain its authority over abuse allegations.
Ratzinger went as far as to slap the wrist of a reporter who dared to ask him about the Maciel investigations. 'One can't put on trial such a close friend of the Pope as Marcial Maciel,' Ratzinger said on another occasion.
Campaigners say Ratzinger's actions show he was prepared to use every means possible to ensure that abuse allegations were not investigated by authorities outside the church.
'We know that in the past few decades the church has tried to keep the abuse issue hidden,' said Mary Grant, a director of a support group of men and women called Snap: the Survivors Network of those Abused by Priests.
The organisation is now seeking an urgent meeting between alleged victims and the new pontiff. 'Ratzinger himself has preferred to dismiss the allegations as a media creation rather than address them,' added Grant.
But a report by the Catholic Church itself estimated that some 4,450 of the Roman Catholic clergy who served between 1950 and 2002 have faced credible accusations of abuse.
Another study, produced last year by the John Jay College of Criminal Justice on behalf of the United States Conference of Catholic Bishops, reported that there had been 10,667 victims of abuse over the last 50 years in the US alone. Campaign groups say the true figure may be significantly higher, especially in developing countries, where priests known to have committed abuse in the west were quietly despatched by bishops desperate to avoid scandal. In addition, abuse claims in developing countries are more likely to have gone unreported, while deference to priests is such that their actions can go unchecked.
Inevitably, lawyers have had a field day as a trickle of early abuse claims that surfaced in the Eighties became a flood in the Nineties. Already it is estimated the church has paid out hundreds of millions of dollars globally to settle claims. Snap estimates that anything between $400 million and $1.3 billion has been paid to victims, and that the final estimate could be as high as $5bn.
The Catholic church's reputation has been damaged worldwide. In the US a 16-month investigation by the Massachusetts Attorney General's office concluded in 2003 that Roman Catholic priests and church staff in the Boston archdiocese molested more than 1,000 children over six decades.
Last February, Dr Kathleen McChesney of the US Conference of Catholic Bishops warned that many more cases were yet to come to light in the US. 'In 2004 at least 1,092 allegations of sexual abuse were made against at least 756 Catholic priests and deacons in the United States,' McChesney said.
Two years ago the Irish government revealed that the bill to compensate victims of sexual abuse by clergy could total €1bn.
Cardinal Cormac Murphy O'Connor, leader of the Catholic church in England and Wales, was forced to appoint child protection representatives in every parish in a bid to quash criticism that it had ignored abuse claims. Between 1995 and 2001, 21 Catholic priests were convicted of sexually abusing children in England and Wales. Murphy O'Connor himself was attacked for failing to report allegations against Michael Hill, a priest in his charge when he was Bishop of Arundel and Brighton. Hill was convicted of abusing nine children.
Now, with a new Pope in the Vatican, the hope for the abuse victims is that the church will start to investigate their allegations. 'I have mixed feelings about Cardinal Ratzinger becoming Pope,' said Barbara Blaine, founder and president of Snap. 'He has been so slow to deal with the Maciel case, but then if he brings the qualities that earned him his reputation as a papal enforcer to addressing the sex abuse scandal, that will be a good thing.'
In what is likely to be seen in some quarters as a clear sign that the Vatican believes it must now confront the abuse scandal, The Observer has learnt that, earlier this month, shortly before he was elected Pope, Ratzinger despatched special envoys to several cities across the globe to take testimonies from Maciel's alleged victims in confidence.
His reasons for revisiting the case, though, remain unclear. One theory is that Ratzinger learnt that confidential evidence will soon spill into the public domain and that he has decided to act ahead of this. Others suggest that he initiated the investigation for political reasons, to help boost his chances of being elected Pope.
What is irrefutable though is that, shortly after he was elected the spiritual head of more than one billion Catholics last week, Ratzinger approached Cardinal Francis George, archbishop of Chicago. The last time the two men talked, George raised the abuse crisis with Ratzinger and pressed him to intervene.
In perfect English, the new Pope assured George that he remembered their last conversation and would 'attend' to the matter. The reign of Benedict XVI may well be judged on whether he holds true to his word.