The Vatican has arrested two members of a commission set up by Pope Francis to advise him on economic reforms on suspicion of leaking secret documents.

Monsignor Lucio Ángel Vallejo Balda, a high-ranking Spanish clergyman, and Francesca Chaouqui, an Italian laywoman and public relations expert, were arrested after a months-long investigation into the dissemination of “news and confidential documents” to two writers of forthcoming books, the Vatican said. 

No further information about the nature of the documents was released. The Vatican described the leaks as a “serious betrayal of the trust bestowed by the pope”.

Balda and Chaouqui were members of a special commission appointed to address possible economic reforms of the Vatican. It has since been disbanded.

Vallejo Bald, 54, is believed to be the highest-ranking member of the Vatican’s central bureaucracy, known as the Curia, ever to have been arrested.

Chaouqui, 33, is no stranger to controversy. Eyebrows were raised within the Vatican following the discovery of tweets sent from her account before she was hired that criticised Vatican officials and called the former Vatican secretary of state, Tarcisio Bertone, “corrupt”.

After being nominated to the post in 2013 she published “artistic nudes” of herself online, according to a report in the Italian magazine Panorama, which subsequently disappeared from the internet.

The Vatican said both officials had been detained following interviews at the weekend and Chaouqui had since been released, reflecting in part her cooperation with the investigation.

“Publications of this kind do not contribute in any way to establish clarity and truth, but rather to create confusion, and partial and tendentious interpretations,” the Vatican said. “We must absolutely avoid the mistake of thinking that this is a way to help the mission of the pope.”

The twin arrests came days before two Italian authors are due to release books that their publishers say will reveal new evidence of scandals in the Vatican and alleged conspiracies by the old guard to undermine Francis’s reform efforts.

They were the first such arrests since Paolo Gabriele, Benedict’s butler, was detained in 2012 for stealing documents from the pope’s desk and leaking them. Those leaks included letters from Vatican officials complaining to the former pope about alleged corruption in the Holy See.

One of the two books due to be released on Wednesday is Merchants in the Temple, by the Italian journalist Gianluigi Nuzzi, whose 2012 book His Holiness was based on leaked documents he received from Gabriele. Gabriele was convicted and served several months in the Vatican jail before Benedict pardoned him and he was released. He is now working in a Vatican-run hospital. 

The other book, Avarice, is by another Italian journalist, Emiliano Fittipaldi.

The Vatican said its police had been investigating the disappearance of documents for the past few months. The statement accused the two Italian authors of trying to reap advantages from receiving stolen documents, saying this was “a gravely illegal act”.

It is the third time this year that the Vatican has had to deal with leaks. In June the pope’s landmark encyclical on the environment was leaked before publication, and last month a private letter from 13 conservative cardinals complaining about a meeting of bishops on family issues was published by an Italian magazine.