Health Secretary Alan Johnson did not challenge Gordon Brown for the Labour leadership because he felt he was "not good enough" to be Prime Minister.
Speaking to BBC Radio 4's Desert Island Discs, Mr Johnson - who was long touted as a possible successor to Tony Blair - ruled out any future bid for the leadership and accepted that Cabinet minister was the highest rank to which he could aspire.
In a deeply personal interview, the former union boss spoke frankly about the poverty and violence of his childhood and how he "celebrated" when his father left home.
Mr Johnson, who was brought up by his "incredible" teenage sister Linda after their mother died, recalled the day he came home to find his father gone.
He told interviewer Kirsty Young: "We celebrated. This was a happy day for me and my sister. I'm not sure how my mum felt about it. He had gone and had cleared out - we were pleased about that, me and my sister.
"I don't think he was a very pleasant person."
Mr Johnson described his days as a sharply-dressed London mod who was a married father-of-three and a postman by 20, then rose to lead the Communication Workers Union, become an MP and Cabinet minister and be tipped as a future Prime Minister.
Asked why he did not mount a challenge for the Labour leadership earlier this year, when many commentators saw him as the most likely Blairite candidate to stand in Mr Brown's way, Mr Johnson was blunt in his assessment of his own limitations.
"I don't think I would have been good enough, frankly," he said. "I don't think I've got the capabilities.
"You get to a level and look around and think 'Perhaps I could go to the next level'. I don't think I could go to that level, which is the only level up from being a Cabinet minister."
