The fallen CIA director said he regretted the liaison with Paula Broadwell that has thrown America's national security establishment into crisis, but insisted that he did not pass her classified intelligence.
Describing his wife, Holly, as "far better than he deserves", Mr Petraeus said in an off-camera conversation with a television journalist that he was guilty of a "personal failing" and had "engaged in something dishonourable".
"He sought to do the honourable thing in response," recounted Kyra Phillips, a reporter for HLN, "and that was to come forward. He was very clear that he screwed up terribly."
Mr Petraeus, who has not been seen in public since his resignation a week ago, is due to testify early Friday morning to congressmen investigating the Sept 11 attack on the US consulate in Benghazi, Libya.
The retired four-star general rejected conspiracy theories that his downfall had been engineered to somehow prevent him from disclosing information on the attack that could be damaging to President Barack Obama.
"He said this has nothing to do with Benghazi, and he wants to testify," said Phillips. "He will testify."
The 7.30am (EDT) hearing on Friday, by the House intelligence committee, will be closed to the public.
Senior Republicans, including Senator John McCain, accuse the Obama administration of failing to provide adequate consular security despite requests from Ambassador Chris Stevens, who was killed in the attack.
They also allege that the White House deliberately misled the public by stating initially that the attack had spilled over from a chaotic protest against an anti-Islamic American film, when in fact it appears to have been a co-ordinated terrorist strike by jihadists.
Member of Congress were last night shown grainy CCTV footage from the consulate, which Republicans said vindicated their claims.
Fears that Mr Petraeus had briefed Mrs Broadwell on secret details of the Benghazi assault were sparked by the emergence of footage of a speech in which she said the attackers had been attempting to spring Libyan radicals from a makeshift CIA jail. Authorities said this was not true and that she had misinterpreted media reports.
In his conversation with Phillips, Mr Petraeus reiterated assurances given by Mr Obama in a press conference earlier this week "that he has never passed classified information to Paula Broadwell".
Eric Holder, the US attorney general, appeared to back Mr Petraeus in a press conference yesterday, saying he “felt very secure” that the affair had not led to any national security breach.
The FBI, which searched Mrs Broadwell's home in North Carolina for several hours this week, is understood to have found classified material from as-yet unknown sources on her laptop computer.
However reports claimed last night that not only do the documents date from before Mr Petraeus’s time at the CIA, but that none of the material comes from the agency.
Mr Petraeus, 60, resigned last Friday after confessing to an affair with Mrs Broadwell, a 40-year-old author and former military officer, which was being investigated by agents from the FBI.
The inquiry was prompted after Jill Kelley, a 37-year-old "social liaison" for the US military in Florida, received anonymous emails warning her to stay away from Mr Petraeus, whom she says was a friend.
Mrs Kelley's complaint to the FBI led agents to email accounts owned by Ms Broadwell, and to the discovery that she was engaged in a relationship with Mr Petraeus. They also discovered multiple affectionate emails exchanged by Mrs Kelley and Gen John Allen, the commander of US forces in Afghanistan.
Mr Petraeus told Phillips that since news of their relationship emerged last week, he had not spoken to Mrs Broadwell, who is staying in Washington and has had her military security clearance revoked.
An aide also claimed Mr Petraeus had not been following media coverage of the saga. "He wants to maintain a distance and focus on his family at this time," said Col Peter Mansoor.
The CIA’s internal watchdog announced last night that it was beginning an investigation in Mr Petraeus’s “general conduct” during his time as head of the agency.
The soap opera at the top of the US military yesterday prompted Leon Panetta, the US defence secretary, to order a review of the training in ethics given to senior officers.
Ordering Gen Martin Dempsey, the chairman of the joint chiefs of staff, to assess the adequacy of current ethical training, Mr Panetta said that recent evidence of misconduct by senior officials had "the potential to erode public confidence in our leadership".
"Any behaviour that negatively impacts our ability to perform that mission is unacceptable," he added. Mr Panetta is due to send an interim report to Mr Obama by December 1.
He has frozen Gen Allen's nomination to become NATO commander, pending the completion of a full investigation into his contacts with Mrs Kelley.
However he said yesterday: "I don't think anybody ought to jump to conclusions".
Reports differ about the scale and content of contact between Gen Allen and Mrs Kelley. Officials have briefed reporters that there are tens of thousands of messages, some sexually explicit. Yet allies of the general insist they exchanged only dozens of emails, which were merely friendly.