It is the degree of choice for the Westminster elite, claiming six cabinet members and three Labour leadership contenders among its alumni. Why does Oxford's politics, philosophy and economics course dominate public life?

In the corridors of power, at the very highest reaches of government, a form of educational freemasonry holds sway.

It has nothing to do with Eton College, nor even the Bullingdon Club - both far more commonly-cited lightning rods for resentments about class, privilege and the fast track to power.

Instead, the surest ticket to the top - for Conservative, Labour and Lib Dem politicians alike - is surely a degree in politics, philosophy and economics (PPE) at the University of Oxford.

No fewer than six members of the cabinet, including the prime minister, foreign secretary and chief secretary to the treasury, are Oxford PPE graduates, as are an additional two ministers who attend their meetings.

Where an Oxford PPE gets you Conservative: David Cameron (prime minister), William Hague (foreign secretary), Jeremy Hunt (culture secretary), Philip Hammond (transport secretary), David Willetts (universities minister), Sir George Young (leader of the Commons)

Lib Dem: Danny Alexander (chief secretary to the Treasury), Chris Huhne (energy and climate change secretary)

Labour: Ed Balls, David Miliband, Ed Miliband (leadership candidates), Lord Mandelson (former business secretary), Jacqui Smith (former home secretary), Ruth Kelly (former transport secretary), James Purnell (former work and pensions secretary)

Labour, for all its egalitarian rhetoric, can hardly claim an advantage. As ballot papers go out to the party's members for the leadership contest, three of the contenders for that crown - David and Ed Miliband, plus Ed Balls - are alumni, as are such big names from Gordon Brown's government as Lord Mandelson, Jacqui Smith, Ruth Kelly and James Purnell.

Indeed, in the present House of Commons there are believed to be some 35 Oxford PPE-ists, compared with 20 Old Etonians.

It is a tradition that stretches back decades. Harold Wilson, Edward Heath, Shirley Williams, Edwina Currie, Barbara Castle - all left their mark on politics in different ways, but all started out with an Oxford PPE.

That Oxbridge graduates in general make up a disproportionate number of the nation's elite is, of course, hardly news, as is the fact that UK politicians of all parties are drawn from a narrow educational base compared with the rest of society.

But what is it about this one course in particular that it holds such an apparently indomitable grip on the highest echelons of power?

A degree defined by breadth rather than depth seems tailor-made for the Westminster system and its regular reshuffles, in which front-line politicians can be running the prison service one day and attempting to steer the economy the next while aspiring to the grand diplomacy of the Foreign Office.

The very title of the course itself conjures up an image of each student as some kind of civic ubermensch, a combination of Machiavelli, Mary Wollstonecraft and David Ricardo.

The politician's view David Heathcoat Amory, former Foreign Office minister and MP for Wells from 1983 to 2010 "I got into Oxford to read sciences, but I changed after a year. I liked the mix between the academic and the practical. "For me, the most useful thing was the philosophy. It turned me into a sceptic and changed my thinking for ever. "We should be pleased that our leaders are well educated. "I think it's something we should celebrate - there are worse alumni to be governed by."

But Observer columnist Nick Cohen, an Oxford PPE graduate, says he now regrets switching to this "silly degree" from history while an undergraduate.

He notes that, while the influence of the École Nationale d'Administration on producing public servants is a subject of regular controversy in France, the scope of Oxford's PPE department receives relatively little scrutiny.

"It's a degree for generalists, and British society has always loved generalists," he says. "But I think we'd certainly benefit from more scientists and engineers at the top.

"It's far easier to condemn Eton or the failure of the comprehensive system. But I went to Oxford, Christopher Hitchens went to Oxford, Ian Hislop went to Oxford - who are the people who are going to eviscerate the phenomenon?"

Indeed, journalists are almost as well-represented as statesmen and women among well-known PPE alumni, not least at the BBC. Political editor Nick Robinson, economics editor Stephanie Flanders and the Today programme's Evan Davis are all graduates (full disclosure: the present author studied politics at Edinburgh).

Nonetheless, few would deny that competition for a PPE place is fierce, and that the course itself imposes a rigorous workload.

Students typically must endure two tutorials a week, in which they present a paper and are grilled on it intensively - such sessions having a ratio of just one or two undergraduates for each academic.

The student's view Tabassum Rasheed, 19, has just completed the second year of her PPE degree at Oxford. "I can see why so many people who want to get into politics do this course. It teaches you how to argue properly, especially through philosophy options such as rhetoric. "The tutorials really sharpen up your thinking - you're arguing your point of view with the person who wrote the books you're studying. "You can do what you want with it, too - personally I dropped the economics after second year and specialised in political theory, the politics of the middle east and so on. But I have friends who specialise in social policy, others in economics. "I wouldn't say it's dominated by people who want to be politicians. I'd like to travel and work for something like Amnesty International or Human Rights Watch, and a lot of PPEs go into the City or do law conversions."

Professor Iain McLean, who taught the course from 1978 to 1991 and counts Nick Robinson among his former charges, says the breadth of the subject matter covered and the fact that students are constantly challenged to justify themselves prevents any danger of "group think".

"It's fundamental to the teaching method to be Socratic - it's our job to ask questions and encourage analytical thinking," he says.

"It was designed to be deliberately broad. Because it's interdisciplinary, we can speak across subject boundaries."

Indeed, PPE's introduction in the 1920 - initially under the title "modern greats" - was designed to offer an alternative to classics for scholars hoping to enter the civil service.

From their second year onwards students are offered a greater degree of specialisation and the opportunity to focus their interests.

The journalist Toby Young, who read PPE at Brasenose College two years ahead of David Cameron, is a defender of the course and believes it offers a firm intellectual grounding for would-be leaders across the political spectrum.

"Among the 10 people reading PPE at the same time as me at Brasenose, you had everything from Monday Club fascists to revolutionary Marxists, plus every shade of opinion in between," he says.

"But when I went on to study at post-graduate level at Harvard, everybody was a liberal. One of the hallmarks of PPE graduates is that they are quite independently minded."

But the question remains: why does this course in particular dominate cabinet tables, rather than similar programmes at Cambridge - or, for that matter, the PPE degrees offered by the universities of Durham, Lancaster and York?

Geoffrey Evans, a fellow in politics and a senior tutor at Nuffield College, Oxford, acknowledges that the course's reputation for producing top-rank politicians is self-perpetuating, with the "the elite frog pond of Oxford" proving a strong lure for students with the means and wit to get through the door in the first place.

"They are pretty bright too, it is fair to say - though they will in the main have had advantageous circumstances in which to cultivate that brightness," he adds. "And ambitious - many no doubt see such positions as a natural outcome of their social and educational opportunities, and the circles in which they mix will in general hold lofty expectations as to what constitutes a suitable occupational outcome.

"All in all, it's how the class system works."

Maybe so. Either way, the political battles of the future seem at least as likely to be fought first in the Oxford quads where PPE is taught as the playing fields of Eton.