The idea that equal opportunities have declined in Britain over the past 40 years - as a new report from the Sutton Trust shows - is one that most people find hard to grasp. Everyone knows of people from humble backgrounds who go to university and become company directors, judges or doctors.

But the truth is that it is now harder for a child from a lowly social background to reach the top than it was in the 1950s and 1960s. This is largely because access to professional careers is now almost wholly dependent on educational qualifications.

Even industries such as the media, which lack formal entry barriers and require no large body of technical knowledge, have become almost impossible for non-graduates to enter. And, despite the growth in universities, children from affluent homes have a far better chance of acquiring degrees than those from poor homes. Fewer than one in 10 from the poorest fifth of the population have degrees against nearly one in two from the richest fifth.

This is one of the great ironies of our age. Education, long seen as the key to achieving equal opportunity and social mobility, has become the great barrier to it. When only a minority got qualifications, everybody else, rich or poor, left school on a level playing-field. Now we have a hierarchy of qualifications, right up to those acquired on professional courses beyond first degree level.

At every level of the hierarchy, those from monied homes are at an advantage. In effect, we have gone back to the early 19th century when your family’s money could buy a position in the Army, Church or civil service.

Then the desired position was bought directly; now it is bought indirectly.

Sending your child to a fee-charging school, buying into the catchment area of a “good” state school, financing your 21-year-old through postgraduate professional training or unpaid work experience - all these will secure qualifications and opportunities and, through them, professional careers.

As the Sutton Trust report shows, the United States also has low social mobility. This is remarkable because equality of opportunity is part of the American myth and there, too, education was seen as its main instrument.

Even more remarkably, given that the American dream is founded on the idea that enterprise and hard work can get you anywhere, the link between qualifications and earnings is exceptionally strong. Home background still has a big influence on the chances of getting a qualification in the first place, but less so than here.

This, and other findings from the Sutton Trust report (carried out by a team from the London School of Economics), leave me more convinced than ever that UK and US comprehensives are not to blame, as many would like us to believe. The study shows that Canada, Sweden, Norway, Denmark and Finland all have higher social mobility than Britain and the US (this may also be true of Germany, but the sample was too small to draw firm conclusions).

All these countries have fully comprehensive systems and, within those systems, less of a pecking order of schools than we do. OECD studies suggest Finland in particular has a high degree of social mixing in its schools. It is worth adding that these five nations are generally more successful economically than Britain.

The lot of children from poor homes can be improved in all kinds of ways, not least by pouring money into education and care in the early years. But for compulsory education, the biggest policy priority should be to create, in each school, a mix of abilities and home backgrounds that comes as close as possible to the mix in the general population. Unfortunately, the policies proposed in the election campaign by both the Tories and new Labour would move us in exactly the opposite direction.

Peter Wilby is editor of the New Statesman.