The news comes days before a Vatican judge is expected to rule on whether Paolo Gabriele will stand trial.
"We expect a formal charge," lawyer Carlo Fusco told Italian news agency ANSA. The Vatican has said the decision over whether Gabriele will be charged with "aggravated theft" is expected some time this week.
The 46-year-old was arrested in May during an investigation into the leak of private papal documents to the media. He was held for 53 days in a Vatican cell before being put under house arrest in July to await the judge's decision.
Fusco said his client had acted alone, denying reports that Gabriele is part of a wider whistleblowing operation aimed at shaking up the Vatican hierarchy.