By Philip Pullella

VATICAN CITY (Reuters) - Two members of a commission Pope Francis set up to study reforms, including a high-ranking Holy See official, have been arrested on suspicion of leaking confidential documents, the Vatican said on Monday.

It was one of the biggest internal scandals to hit Francis' papacy so far and was reminiscent of the "Vatileaks" furor that preceded the resignation of former Pope Benedict in 2013.

Spanish Monsignor Lucio Angel Vallejo Balda, number two at the Vatican's Prefecture for Economic Affairs, and Italian laywoman Francesca Chaouqui, a public relations expert, were arrested over the weekend, a Vatican statement said.

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

Chaouqui, 33, whose sexy photo of herself on her Facebook page raised Vatican eyebrows when she was appointed to the commission in 2013, was released on Monday after she agreed to cooperate with the investigation, it said.

The Vatican said the leaks represented a "serious betrayal of the trust bestowed by the pope", without providing any details. There was no immediate comment from Vallejo Balda and Chaouqui, or their lawyers.

Both were members of a commission that Francis set up shortly after his election to advise him on economic and bureaucratic reforms in the Curia.

The committee completed its work last year and handed its report to the pope, who subsequently made some changes in Vatican administration, including the establishment of a new economic ministry.

The twin arrests came just days before two Italian authors were 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' reform efforts.

SHADES OF "VATILEAKS"

They were the first such arrests since Paolo Gabriele, Benedict's butler, was arrested in 2012 for stealing documents from the pope's desk and leaking them.

Those leaks included letters to the pope 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 Italian journalist Gianluigi Nuzzi, whose 2012 book "His Holiness", was based on the 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, called "Avarice," is by 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".

Such books "generate confusing and partial, tendentious interpretations", the statement said. It added that the Vatican might ask Italian authorities to take unspecified action against the two authors.

It was 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.

(Reporting by Philip Pullella; Editing by Crispian Balmer and Tom Heneghan)