The official spoke on condition of anonymity in line with court policy.
Olmert is on trial for a series of offenses allegedly committed while he served as mayor of Jerusalem and later as a Cabinet minister. The charges were filed only after he was elected prime minister in 2006 and forced him to resign three years later.
Olmert is facing charges in three separate cases: Illegally accepting funds from an American supporter, double-billing Jewish groups for trips abroad and cronyism. Olmert has denied any wrongdoing.
If convicted on all the charges, Olmert could face five years in prison.