The Vatican has arrested a Spanish prelate and a social media expert for allegedly stealing and leaking classified documents in the second such scandal to hit the secretive institution in three years, AFP reports.

Monsignor Lucio Angel Vallejo Balda, 54, who served on a special commission set up by Pope Francis to advise him on economic reform within the Vatican, was arrested along with a second member of the commission, Francesca Chaouqui.

The arrests were part of a several months-long investigation into the "misappropriation and disclosure of classified documents and information".