As individuals, they could hardly seem more different: one a conservative, traditionalist, church-going Anglican; the other a liberal, louche, drinking-and-smoking atheist. So when the self-styled reactionary right winger tackled the fellow-controversialist with diametrically opposed views, opening up a dormant feud that goes back decades, it was always going to get heated.

That the pair in question are bitterly divided brothers only served to raise the temperature last week when Mail on Sunday columnist Peter Hitchens went on the attack in a review of his elder sibling Christopher's new anti-religion book, God is Not Great: How Religion Poisons Everything .

"People sometimes ask how two brothers, born less than three years apart, should have come to such different conclusions," wrote Peter, adding enigmatically: "To which I'd answer that I'm not sure they're as different as they look, and that it's not over yet."

Other than an acknowledg-ment that, "[Like] everything Christopher writes, it is often elegant, frequently witty and never stupid or boring", Peter's review was damning, and even questioned whether Christopher truly believed his own assertion that there isn't a God.

Peter, 55, confirms to me that he was implying in his review that Christopher, 58, was closer to religious belief than he had ever accepted. "There is always, in the atheistical struggle with God, the fight against temptation. If it didn't matter to you, why write a book about how wrong it is? The first person you have to convince with any book you write, is yourself. If you didn't need convincing... why go to all those lengths?"

On a book tour in Los Angeles, Christopher agrees to read the review by email and then flatly rejects the idea that he is, as he put it, a " repressed seeker". Prefacing his response to his brother's review with faint praise - "a quite stirring and eloquent piece" - Christopher says: "The sickly idea that this interest is a disguised cry for help... only demonstrates the insecurity and the bad faith of the godly." The elder brother then adds: "Though I slightly dislike to say this, [Peter] offers himself as yet another example of how the religious mentality forces honest and reasonable people to say dishonest and irrational things."

This exchange is merely the most recent of a long line of clashes between two brothers who make up a unique phenomenon in the world of journalism. Normally, they tend to pass each other by, partly because they operate on different sides of the Atlantic, with Christopher based in Washington. " Quite a lot of people who read Christopher don't know that I exist; and quite a lot of people who read me don't know he exists," Peter says. "We live in different worlds."

Once, however, they were something close to brothers in arms. They grew up in boarding schools around England and Scotland, constantly on the move thanks to their father's postings as an officer in the Navy. In their early teens Christopher led Peter into an interest in radical politics, and both spent their university years (Christopher at Oxford, Peter at York) fighting for the International Socialists, something of a minority within a minority: a Trotskyist faction.

As well as fellow Trots, they both became journalists (and later polemicists), with Christopher doing a stint at the New Statesman with friend Martin Amis, and Peter joining the Daily Express as what he describes as "the world's worst general reporter". At one point, Christopher went to write leaders for the Express, where the brothers were occasionally mistaken for one another.

But Christopher soon moved to America to cultivate his status as the quintessential Englishman abroad, writing prolifically for a wide range of publications including left wing journal The Nation, and gradually beginning a side-career of controversial television appearances. As an opponent of the previous Gulf War, he shocked viewers by challenging right-wing actor Charlton Heston to name the countries surrounding Iraq (Heston could not) before telling him to "keep your toupee on".

Peter stayed at the Express, where he was education correspondent before becoming an industrial correspondent, in the days when union stories were dominating the front pages, and then deputy political editor. Moving abroad, like his brother, he reported from Moscow during the end of the Cold War, then Washington, before finally returning to become the paper's star columnist. When the paper was bought in 2000 by Richard Desmond, who published a raft of pornographic magazines, Peter left for the Mail on Sunday, where he has further established himself as one of Britain's best-known columnists on the political right.

Christopher, meanwhile, always knew he wanted to emigrate to the States – as much as he always knew he wanted to write – and has recently been made a US citizen at a time when he says he feels more comfortable politically as well as culturally in his adopted country. Christopher believes that the US is "on the right side of history" these days – having clamoured for the invasion of Iraq – unlike during the Vietnam war, which he still opposes.

While Christopher was for decades to remain on the radical end of the spectrum, his brother moved to the opposite extreme. Peter's "conversion " is believed by Fleet Street myth to have come on a cold night in 1975. Legend has it that he underwent a "religious experience". But the truth is less simple. What is known of that evening is that Peter was selling Socialist Worker on a very wet train platform in Swindon when he finally decided to turn his back on the left. He says now that it was, ultimately, a choice that had to be made "between socialism and liberty". But it was a more gradual process than some think, and he himself invented the sudden conversion story as a cover to fend off constant questions on the subject.

There is a similar, more recent, and perhaps too-simplistic "conversion" narrative concerning Christopher, who was once the toast of America's liberal elite and now the scourge of what he calls the "status quo" left, whom he moved away from after September 11th. He has plunged his energies into supporting the coalition's "war on terrorism".

As late as September 2003 Christopher was still describing himself as a "Marxist". He has now emerged as a leading "neo-con" hawk, having famously resigned from The Nation in disgust that, as he claimed, its writers and readers saw George W Bush as a greater threat than Osama bin Laden.

Unlike Peter, Christopher denies any talk of a "conversion", pointing out a consistency in his thought, especially his hatred of religious fanaticism. In his new book, though, he does say: "Sometimes I miss my old convictions like an amputated limb."

Peter, having long urged his readers to oppose ID cards if they don't want to be "registered like a child molester or a cow", also opposed the invasions of both Afghanistan and Iraq, siding with small-c conservatives such as Matthew Parris and Kenneth Clarke, who feared for the danger to and lack of self-interest for Britain.

With characteristic eccentricity, Peter even once described the Iraq invasion as "a left-wing war". For Christopher, "It was classic Peter – mad, but with a logic to the madness". In response to this, completing one of their most fraternal exchanges, Peter said: "Classic Christopher – spiteful, but with a logic to the spite".

Now Christopher's friendship with and defence of the former World Bank boss Paul Wolfowitz, and his disdain for what he termed "Islamic-fascism" – a phrase later used by President Bush – has won him new friends on America's right, and caused many bitter rows with old comrades on the left.

Peter, who knows a bit about shifting one's politics, said recently of his brother: "He may have entered the really wobbly phase of mind-changing, where the floor suddenly gives way beneath you without warning and you either have to deceive yourself – the safer thing – or acknowledge that you have been wrong and – worse – wrong in public".

However, it doesn't appear that Christopher will be changing his mind any time soon on religion – the brothers' biggest public dividing point.

Peter had never previously reviewed any of Christopher's work, but, he tells The Independent after his scathing piece is published, he chose to intervene on this subject because Christopher has repeatedly identified it as the issue on which they most disagree.

That is putting it mildly. In a 2002 interview Christopher made the extraordinary – now withdrawn – announcement that he would not look after Peter's children if something happened to his younger brother and, citing religion, as opposed to politics, as the key difference between them.

He said: "I can't be hypocritical about it. I'm sure he would, but I wouldn't. And anyway, Peter would want his children to be brought up as Christians... Politically the differences are trivial, but I have a bigger difference between myself and anyone who believes in religion than I do in any other subject. I don't trust anyone who believes in religion. So we don't agree".

During the same frank interview, Christopher revealed that after he discovered his mother died in mysterious circumstances – apparently a suicide pact with a boyfriend in Athens – he found a note his mother had dressed only to "Christopher". He has since been quoted as saying, "If you were the mother of Christopher and Peter, who would be your secret favourite?"

The divide, then, is real, and it is fuelled by lasting, deep disagreements over matters on which it is in their nature to disagree. But it is also in their blood to turn on their own "sides". When Peter recently attacked the Tory party, and David Cameron's leadership in particular, he shocked – and offended – many of his own cult of fans and followers.

Those who knew Peter however, were unsurprised. A former leftist, Peter was never comfortable with some of his new allies on the right. He had frequently told friends he preferred Labour conferences to Tory ones, partly at least because the people were nicer and better company. But there was another, deeper factor, which provides context for Peter's antagonism towards the Conservatives: it fits into a life-long Hitchens-esque pattern, pioneered by his older brother, of fierce independence.

There are similarities that go beyond looks, voice, and even their mutual use of controversialist stunts to sustain their profiles. They share, for example, a love of argument, and a disdain for mainstream partisan politics and conventional wisdom. "You won't find either of us careerist boot-lickers or attached to any political party", Peter says. " Neither of us is in the mainstream or ever will be".

But if the brothers are similar in tactics and style, they certainly differ in person as in politics. Peter's highly intelligent wife, Eve, stays at home to perform what Peter describes as "the most honourable job in the world" – bringing up her side of the next generation of Hitchenses (they have three children, all of whom were brought up with strict anti-TV guidelines).

Christopher – whose lifestyle with second wife Carol Blue was recently described in a recent profile in The New Yorker as that which "a spirited thirteen-year-old boy might hope adulthood to be"– has, until recently at least, been considered in bien-pensant circles as the more fashionable Hitchens brother.

Peter is dubbed "Bonkers" in opinion columns by his colleagues in the British press. This tactic, he says, is the old Soviet trick of dismissing outsiders as madmen. He has long been viewed as a right-wing monster, the kind you'd never have round the house, for fear of him saying something offensive. Like many assumptions about the Hitchens brothers, it is false. He is a loner, but the fact that Peter may actually be likeable is one of Fleet Street's best kept secrets.

Similarly, myths surround Christopher, chief among them that he's a hopeless drunk. While he has admitted that he regularly drinks enough to "stun or kill a mule", he defends his professionalism fiercely, pointing out that he has "never missed a deadline".

In so many ways, politically, literally, religiously, the Hitchens brothers are miles apart. Occasionally they come together and clash in public. But whichever way the ever-changing, unpredictable new world order spins, it's a safe bet that they at least, will remain permanently in opposition.

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