In the past three years, Opus Dei has risen in profile from an obscure Catholic cult to worldwide recognition, thanks mainly to its starring role in the bestselling novel The Da Vinci Code.
It has recently tried to readdress its reputation as a secret and sinister organisation by setting up a website and press office and encouraging members to speak about their faith.
But it has still not managed to shake off rumours of brainwashing, homophobia and murky political influence, alongside derision at practices such as the self-flagellation demanded of its members.
Opus Dei - meaning "the work of God" - was founded in 1928 by a 26-year-old Spanish priest called Josemar a Escriv, who was apparently driven by divine inspiration. The movement was based on the idea that work is a God-given devotion and that spirituality could be practised in everyday life rather than simply in church on Sundays.
Members are either "numeraries" who are celibate and live in communities but go out to work, and "supernumeraries" who marry, have children and live normally. All members attend church every day and are encouraged to perform daily acts of unselfishness.
There is also the much-talked-about act of "mortification of the flesh", where followers do penance for their sins by strapping a spiked bracelet called a cilice to the top of their thigh.
In 1950 Pope Pius XII gave formal approval to Catholics who wanted to join Opus Dei, but the ultra-conservative movement appeared out of step with the liberalising tendencies of Vatican policy in the 1960s.
In 1981, Cardinal Basil Hume, the late head of the Catholic Church in England, banned the organisation from working with children amid allegations from former members that they had been brainwashed, forced to sever ties with their families and required to hand over large chunks of their earnings.
But in 1982, Pope John Paul II made Opus Dei a "personal prelature", meaning it was a quasi-diocese that could deal directly with the Vatican and have some influence within the Church. He also put Josemar a Escriv, who had died in 1975, on the fast-track to sainthood - he was eventually canonised in 2002.
The movement is known to have remained ultra-conservative on homosexuality and abortion. Jack Valero, spokesman for Opus Dei in the UK, said: "We do not have a view of our own on homosexuality, we simply follow the teachings of the Catholic Church, which says that sex is for marriage, marriage is between a man and a woman and the child has the right to benefit from the input of both a mother and a father. The Catholic Church would be opposed to civil partnerships in the way they have been enacted in Britain, but it is for each Catholic to work out for themselves."