Former American military commander and CIA chief David Petraeus will plead guilty to illegally providing classified secrets to his mistress, a dramatic fall from grace for a general once lauded as a war hero.

Petraeus, feted in the US as the man who changed the course of the Iraq war, has signed a plea deal and statement "that indicate he will plead guilty" to unauthorised removal and retention of classified material, the Justice Department said on Tuesday.

The outcome marked a humiliating turn for the decorated four-star general who became the most revered commander of his generation over his role in the Iraq war.

But the plea deal will allow Petraeus to avoid a trial that would have shone an embarrassing light on details of his affair and his flouting of secrecy laws.

The misdemeanour charge carried potential jail time but the plea deal recommends two years of probation and a $US40,000 ($A51,148) fine.

According to the Justice Department, Petraeus acknowledged giving eight "black books" he kept as the commander in Afghanistan to his lover and biographer, Paula Broadwell.

The notebooks were meant to serve as source material for her book about the general, All In: The Education of General David Petraeus.

The notebooks included his daily schedule, classified notes, the identities of covert officers, details about US intelligence capabilities, code words, summaries of National Security Council meetings, and accounts of his meetings with President Barack Obama, according to court documents.

The black books contained top secret and "national defence information", it said.

In October 2012, FBI agents questioned Petraeus at CIA headquarters while he was still director. The retired general told them he had never provided any secret information to Broadwell - a lie that he acknowledged in his plea deal.

Passing the sensitive information to Broadwell and then keeping the notebooks at his home clearly violated his legal obligation to safeguard classified information, authorities said.

None of the classified information appeared in Broadwell's book.

Obama named him CIA director in 2011 but he resigned a year later after his affair with Broadwell was exposed.