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 unauthorized removal and retention of classified material, the Justice Department said 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 cast an embarrassing light on details of his affair and his flouting of secrecy laws.

The misdemeanor charge carried potential jail time but the plea deal recommends two years of probation and a $40,000 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 five-by-eight inch 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 sessions, and accounts of his meetings with President Barack Obama, according to court documents.

The black books contained "Top Secret" and "national defense information," it said.

An official Defense Department historian gathered up classified papers that Petraeus had collected while in uniform but Petraeus "never provided" the notebooks to the historian as required.

- 'Classified stuff' -

Instead, Petraeus kept the notebooks in a "rucksack," he told Broadwell in a conversation she recorded.

"They are highly classified, some of them. ... I mean there's code word stuff in there," the general tells her.

Petraeus later emailed Broadwell promising to give her the notebooks and personally delivered them to a residence where she was staying in Washington DC. He retrieved the black books a few days later and kept them at his home.

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, which was published by Penguin in 2012.

Civil liberties advocates said the treatment of Petraeus revealed a double standard by government prosecutors, as intelligence officers who had tried to blow the whistle on wrongdoing had faced much tougher treatment than Petraeus.

John Kiriakou, a former CIA officer who was prosecuted under the Espionage Act in 2012, said: "Both Petraeus and I disclosed undercover identities (or confirmed one in my case) that were never published.

"I spent two years in prison; he gets two years probation."

However, Republican Senator John McCain, a staunch ally of Petraeus, issued a statement saying it was time to consider the matter "closed" as the retired general had apologized for his actions.

Petraeus was given hero status in Washington for overseeing the troop "surge" in Iraq in 2007 and US leaders credited him for salvaging the troubled war effort. He later served as the top US commander in Afghanistan, where his tenure achieved mixed results.

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

When he left his post at the Central Intelligence Agency, Petraeus signed a non-disclosure agreement promising that he had no classified material in his possession.

But the black books were still at his home. And in April 2013, the FBI arrived with a search warrant at the general's residence and found the notebooks in an "unlocked desk drawer" on the first floor of the house.

Petraeus, now a partner in a private equity firm, was once treated as a potential presidential candidate because his celebrity status as a commander.