Devotees of St John Bosco are in shock after it was revealed that a piece of his brain was stolen in Italy by a thief posing as a pilgrim.

St John Bosco, popularly known as Don Bosco, was an Italian Roman Catholic priest who founded the Salesian religious order, founded in the 19th century to help poor children during the Industrial Revolution. He was born in 1815, died in 1888 and was canonised in 1934. Among many Catholics in northwest Italy, the saint is revered as the patron saint of school children, troubled youth, editors and publishers.

According to the church officials, the thief entered the church in Castelnuovo, near Turin, last Friday. He then left with a glass case which contained a small piece of the saint's brain.

Each year more than 600,000 devotees and pilgrims view the reliquary site and often visit the church to pray before the relic kept in glass, behind the altar. On Sunday, pilgrims gathered to pray for the relic's return.

'Shock and outrage surround theft of St John's Bosco's brain,' blared a headline on Crux, a popular Catholic online publication in the United States.

'I invite whoever took it to give it back immediately, without conditions so we can turn this painful page and continue worthily to honour the memory of Don Bosco in his birthplace,' said Archbishop Cesare Nosiglia of Turin in a statement.

St John Bosco is venerated for having dedicated his life to helping deprived, poor children. The religious order which was founded in 1859, has grown to become the second-largest order in the Catholic Church, and according to the Salesian Mission, the order is regarded as the single largest provider of vocational and technical training in the world. It operates more than 70 colleges, 90 clinics and hospitals, more than 330 orphanages and more than 3,200 schools.

Nosiglia is calling for the immediate return of the reliquary and said in a statement that the theft of the relic was something "you never wanted to hear."

'It makes you think of the profound moral misery of someone who would steal a "sign" that's been left and conserved for the devotion and the faith of all,' he said.

Speaking to Crux, Father Ezio Orsini, the rector of the basilica, said the community is 'deeply saddened' over the theft.

'We trust that Don Bosco can touch the heart of whomever committed this act and make them turn around, just as he was able to transform the lives of the young people he met," Orsini said. "We're also sure that while you can steal a reliquary of Don Bosco, as has happened, you can't rob Don Bosco from us and from all the pilgrims who visit this place every day.'