The fire burned in his belly as, aged 10, he watched his parents being arrested. Three years later he spoke at the funeral of a friend executed by the state. Peter Hain became a political activist at a painfully young age, as he witnessed close up the evils of apartheid and the courage of his parents in trying, in their small way, to fight it.

His father hounded out of work, eventually they felt they could fight no more. Leaving the material comforts of South Africa for a life in England, one can feel his sadness. The title of Hain's memoir, Outside In, is well chosen. Wherever he found himself, he never quite fitted in. In his early 20s he became a leading light in the movement to stop South Africa's rugby and cricket tours. He became the regime's public enemy No 1 and, as was revealed later, the object of extensive surveillance by MI5  supervised by Harold Wilson's Labour government.

The accounts of these incidents are disarmingly understated. Hain describes how in 1972 a letter bomb was sent to the family home in Putney by South African foreign intelligence. It was being opened by his 14-year-old sister, Sally. "Recessed into a thick sheet of balsa wood were hideous metal cylinders and terminals with wires protruding," he writes. "We sat transfixed, expecting it to explode, seconds seeming like ages. Yet nothing happened."

The sometimes clunky writing style reinforces the sense of a passionate young man on a journey. That took him, via the Anti-Nazi League and two failed attempts to enter parliament (beaten by a more ostentatious Chelsea supporter, David Mellor), to the seat of Neath in 1991. He clawed his way up, taking a variety of cabinet positions under Tony Blair and briefly Gordon Brown. Some jobs excited him more than others. In the foreign office under his old friend Robin Cook, he flourished, particularly with his work in Africa.

Here was a man with radical veins but frightened to show them. When he occasionally did, such as proffering the heretical thought that the wealthy should pay more tax, he was humiliated by Brown. Hain learned to keep his head down and British politics was poorer for that.

He confined himself mainly to gestures. He describes how on arriving at the Wales office in 2002 he held aloft a statuette of one of his heroes, Nye Bevan. He asked that his framed poster of Nelson Mandela be hung in his ministerial office to sniffy disapproval from the mandarins. He recalled walking up Downing Street for his first cabinet meeting where on the same spot in 1969 he had protested against Wilson.

In a bizarre passage, Hain extols the intelligence services. He describes the many intercepts he signed off. "It was a fascinating task which I undertook with diligent enthusiasm, sometimes also signing or agreeing warrants for one of my cabinet colleagues if they were unavailable." For sure, anyone in his position would have had to do this, but I find his zeal disconcerting. The same goes for his justifications of the Iraq war. At least he is refreshingly honest about his differences with Cook and about his rows with friends and family.

As a political reporter, I saw a fair amount of Hain at that time. I have always liked and respected him, but the difference between his private concerns and public utterances was large. I believe him to be ethical, so the issue was not duplicity, but an unwillingness to rock the boat. He had only just entered cabinet and the leadership was wary of him.

There was no love lost with Brown. He describes how in their meetings, Brown would "glower". He adds: "if you didn't toe his line you got hammered in the media by being briefed against by his team."

When the time came for the big push  the contest for the deputy leadership in 2007  Hain again looked in both directions. He reacquired his radical rhetoric, describing in an interview with me the Bush administration as the most rightwing in US history. He finished a disappointing fifth out of six. He notes: "Despite all my experience and work in the Labour and trade union movement, maybe I still remained a bit of an outsider." A year later he was forced out in a scandal over the funding of his election bid. He laments this episode, while revealing that his wife Elizabeth and close friends had urged him not to stand. "Although they backed me to the hilt, I wish I had listened to them."

That is a telling admission. As he notes in his conclusion, his political "mission" remains unfulfilled. He made his contribution, and continues to do so, but I can't help thinking, as he does, that it could have been more substantial if only he had been less cautious. Still, I would rather parliament had more of his like, even the muted firebrand, than the new generation cloned from thinktanks and party work.

Peter Hain has lived life to the full, which is more than most of our politicians can say.

John Kampfner is the author of Blair's Wars and Freedom for Sale