Who would have thought that 11 years of a Labour government would make Britain more unequal? Yesterday's official statistics show that since 1997, the poor have - in relative terms - got poorer and the rich richer. Inequality in Britain is now at its highest level since it was first measured in 1961. And that is bound to put a dampener on Gordon Brown's attempts to make our society more mobile.

For the more unequal a nation is, the less social mobility it offers. David Cameron likes to claim that Britain is now a genuine meritocracy in which where we are going is more important than where we have come from. But that's simply not true if you look at the underlying figures. Our society is no more fluid now than it was a generation ago - and it is less fluid than it was a generation before that.

They buck you up, your mum and dad, or they muck you up. Either way, in modern Britain, what most determines where you will end up in life is your parents. If they are high-earning, ambitious professionals, the chances are you will be too. If they are poor and unemployed, you have only a small chance of improving your prospects, whatever the talents you were born with.

Britain - along with America - is one of the most socially rigid nations in the developed world. And that is not because it is uniquely difficult for a poor child to do well here. It is because there is so little downward mobility from the top. If your parents are in the top three social classes (out of the seven defined by sociologists), there is a 74 per cent chance that you will be too. It is only the fact that the middle classes have expanded, thanks to the economy generating more white-collar jobs, that some children born into the working class have been able to move up and join them.

Well-off children have an enormous head start in Britain, and the influences work on them long before they even begin school. The brightest poor children drop from the 88th percentile at the age of 3 (meaning that only 12 per cent of their contemporaries score more highly in tests) to the 65th by the age of 5. The least able rich children, meanwhile, move up from the 15th percentile at 3 to the 45th at 5. At that rate, the dim rich kids overtake the bright poor ones in test scores by the time they are just 7.

So it is not just innate ability that determines your fate. While it may - perhaps - be true that, on average, children of parents in intellectually demanding jobs have a higher IQ than those whose parents are poor and unemployed (as Bruce Charlton argued controversially in Times Higher Education), that could not on its own explain the fact that rich youngsters are more than four times more likely than poor ones to go to university.

Nurture seems to matter at least as much as nature. Children of poor parents here don't tend to be given the same intellectual stimulation or the same impetus to achieve. In a survey of 54 developed countries, England and Scotland showed the highest correlation between children's test scores and the number of books at home. Poor children are less likely to be read to, less likely to be taken to museums or the theatre and less likely to display the good behaviour and social skills that are also associated with success in later life.

They are also more likely to have parents who don't particularly value education. Attitudes to education are incredibly important - which is why disadvantaged Indian and Chinese pupils do much better at school than their white or Afro-Caribbean contemporaries from similar backgrounds.

Why are Britain and America (supposedly the land of opportunity) less mobile than other countries? Economists put it down to our high levels of inequality. The more unequal a society, the harder it is to move out of your social class. The distances are greater, for a start. It is no accident that the most socially mobile nations are Scandinavian.

How depressing, though, for Labour ministers that so much has been done to try to increase social mobility here to so little effect. There has been a huge redistribution of money from the middle classes to the poor. There has been extra investment in inner-city schools. And there has been the introduction of SureStart, a scheme aimed at improving the life chances of children from an early age. Yet all Labour has managed to do is stabilise the decline in mobility.

The trouble is that the countervailing forces have been so strong. The more that we move to a knowledge economy, the more employers value educational succ-ess. Jobs that used to be open to non-graduates now expect a degree, and junior employees without one can no longer hope to be promoted into them. Britain and the US have higher returns to education than most other countries, meaning that graduates can expect to earn far more than those who have not been to university.

This is something that middle-class parents understand, and all their efforts are devoted to ensuring that their children go to university - preferably one of the best ones - and end up in a good, graduate-only job. To this end, they work single-mindedly to find a place for their offspring in the best nursery school, the best primary and the best secondary. If they can't afford to go private, they may employ a tutor to top up at home what their children are taught at school. High educational achievement, for girls as well as boys, has become even more of a spur in middle-class families than it was a generation or two ago.

It is hard for poor parents to compete with these dedicated rivals. The working classes on the whole have a smaller (though often closer) network of friends. The middle classes tend to have a wider (if shallower) circle of acquaintances from whom they can get the best advice on schools, universities and jobs, and with whom they can place their children on work experience. They can afford to buy houses in better catchment areas. They have broadband internet access at home, shelves of books and quiet places for their children to study. They can even “help” with coursework.

Then there is what economists call “assortative mating”. We tend to marry others from the same social class. When girls were not so well educated and mothers stayed at home, this made less difference. Now that high-achieving, high-earning men marry high-achieving, high-earning women who often carry on at work after they have children, the advantages for their offspring are greater still - and so is the polarisation of society.

And finally, of course, there is the question of private schools. Yes, state schools have improved in the past ten years. It would be a scandal if they hadn't, given the amount of money that has been poured into them. But private schools have improved at least as fast. They have upped their fees, allowing them to recruit better teachers and build more facilities. The best ones have become far more academically selective - witness the wails of Old Etonians who can no longer get their sons into the school.

We all know the odd privately educated person who ends up as a poverty-stricken failure. But that sort of downward mobility is almost perversely difficult to achieve in Britain. Private schools give children the social skills, the networks and the academic results that pretty much guarantee them the same status that their parents have enjoyed. In many private schools these days, every sixth-former goes on to higher education. After that start in life, it is pretty unlikely that they will be stacking supermarket shelves. As the Sutton Trust has shown, privately educated people still take a disproportionate share of Britain's top jobs.

There is nothing wrong with middle-class parents wanting the best for their children and going all out to achieve it. The left-wing response, led by Ed Balls, Secretary of State for Children, Schools and Families, has been to penalise these parents by introducing school lotteries and banning selection by interview. Rather than dragging them down, though, would it not be better to try to equalise the chances of less privileged children?

IntoUniversity, a charity with three centres in inner-city London, is trying to do just that. It offers disadvantaged youngsters the sort of opportunities and expectations that middle-class children take for granted. From the age of 7, it not only hosts after-school study sessions with tutors, books and computers, it also introduces the idea of university and professional careers to children who might never have contemplated them. They get taken to museums and theatres, take part in debates, do workshops with bankers and lawyers and journalists, and spend a week hosted by a university discovering how learning can be enjoyable.

Many are then paired with a mentor who is already an undergraduate, often from a similar background, who not only helps them study, but also makes university seem as normal an aspiration as it would be for a middle-class child. And the charity also gives help and guidance that their parents can't always offer: on GCSE and A-Level choices, filling in a UCAS form, choosing a course and a college.

It is startlingly successful and has so far sent more than 80 students to university. Ayisha Adedeji, now 19, started with IntoUniversity at primary school. She won straight As in her A Levels and is now studying law and sociology at Warwick. She remembers being taken on a trip to Belgium, ostensibly to learn about the Second World War, but also to help her and her fellow pupils raise their ambitions. “We stuck stickers on ourselves saying ‘I want to be a doctor' or ‘I want to be a lawyer'. IntoUniversity gave me that extra push.”

Andrew Chaplin, a teacher at Walnut Tree Walk Primary School in inner-city Lambeth, recently took his whole Year 6 class to a week run by IntoUniversity. “Every child in the class now talks about going to university and what course they would like to do,” he says. “It is something many of them would never have even considered before.”

So these are the keys: early intervention to stop bright children tailing off before they reach school; high expectations from teachers to keep them on track when they get there; and initiatives such as IntoUniversity to replicate the home environment that middle-class children enjoy.

These things can work wonders. The introduction of really good universal childcare in Denmark in the 1970s doubled the odds of children with ill-educated parents completing the equivalent of A Levels. And a US programme, aimed at disadvantaged mothers while they are still pregnant and sends a nurse to visit them for the first two years of their child's life, has been shown to give the child a larger vocabulary and a higher IQ. A similar scheme is being piloted here.

Gordon Brown and David Cameron can argue over whether the State or the voluntary sector should be helping poor children to aim high. But they both want to extend opportunity more widely. And they must agree that - while they can't buy an Eton education for everyone - the great start in life that they enjoyed as children is a boon that is still spread far too unevenly in Britain.

www.maryannsieghart.com

Times recommends

• Who killed Jade Goody?
• Daisy Goodwin: This is better than real sex
• Foods that can help to prevent swine flu