Pope Benedict XVI's former butler is starting his 18-month prison sentence in a Vatican City cell for the theft of papal correspondence.

But the Vatican's secretary of state's office in a statement left open the possibility of a papal pardon if Paolo Gabriele repents and asks the pope for forgiveness.

The Thursday statement said the deadline to appeal the Oct. 6 conviction by a Vatican tribunal had elapsed. It described as "mild and fair" the punishment given to Gabriele, who used to serve Benedict his meals and helped him get ready for ceremonies. It noted that Benedict, as sovereign head, has the power to pardon.

Dismissing suspicions of a plot, the Vatican said Gabriele acted alone in pilfering the documents and giving them to an Italian journalist who published them.