To those of the school’s 80 pupils who choose to attend, he talks about Jesus and how his example should guide them, conducts Mass, leads a prayer session and finally hears confession.

For a church school, there is nothing terribly unusual in this, except that Father Calduch is a member of the controversial Opus Dei, regarded by some as one of the highest orders of the Catholic church and by others as a right-wing religious sect.

Oakwood, in Purley, is the only school in the country where Opus Dei has a formal presence. It was set up four years ago for pupils aged two to 11 by three Opus Dei members for their children.

And although the school is keen to emphasise that pupils attend Father Calduch’s sessions on a voluntary basis, for many of the parents who choose Oakwood, the priest’s presence is key.

The central theme of Opus Dei is that Christianity should be present in everything a person does, and not just confined to a couple of hours on Sunday.

“If there is a theme at the school,” says Father Calduch, “it is one of Christian values, not necessarily an Opus Dei one, but one which affects everything they do.”

However, Opus Dei has attracted a great deal of suspicion and criticism since it was founded in Spain in 1928. Javier Echevarria, the founder, has been associated with the Franco regime. The group says it has found favour with the present Pope who controversially beatified Echevarria.

According to the Opus Dei awareness network, which elps people adversely affected by the group, members are often encouraged not to tell parents they have joined. Recruitment is often done through “front” groups and controls - such as opening personal mail and handing over part of your income - are not revealed until after joining.

But it is the sect’s practice of corporal mortification which arouses most concern. It ranges from the innocuous, such as the denial of a treat, to self-flagellation with a knotted whip once a week and the wearing of a “celice”, a thigh bracelet with inverted spikes, for two hours each day.

Andrew Soane, spokesman for Opus Dei in the UK, confirmed that both practices do exist but added: “There are very few people who choose to use them. People have different ways to show their devotion to God.”

Oakwood is keen to distance itself from such extreme practices. Head-teacher Ciro Candia said: “There is no formal link to Opus Dei. We use religious textbooks that are used in many Catholic schools, and children have the choice of whether or not to attend Mass.”

Opus Dei does not receive any income from the pound;1,400-a-year fees, he said.

While Oakwood is the only UK school with Opus Dei connections, the organisation does run 20 youth clubs for secondary pupils - in Manchester, Oxford, Glasgow and London. At 18, youngsters can decide whether they want to become Opus Dei members.

Rachel Storm, acting director of religious information organisation, Inform, said: “Opus Dei has had quite a controversial past. But the group itself is quite strict in informing parents, especially with young recruits who are encouraged to seek parental consent.”