A strong correlation between pupil performance and their parents' social-economic status leaves England's schools system lagging behind the world's best education systems, a Government commissioned research study has found.
The study, by global consultancy firm McKinsey, compared England against about 20 high-performing education systems and found that it needed to strengthen the consistency of classroom teaching.
It cites the fact that pupils eligible for free school meals are less than half as likely to get five or more A*-C grade GCSEs compared to other pupils.
The report was released just days before the Conservatives launched an offensive against the English education system failing the poorest members of society. It also says that academic content and standards in England's schools are not fully meeting the demands of employers and universities.
But it notes evidence that variation in pupil performance lies mainly within schools rather than between them.
The report rates most aspects of England's system as good or world class, although high expectations for pupil achievement - "a key attribute of high-performing systems" - are only rated as fair.
"Many school reforms implemented in England are shown to be world-leading," it says. "However, they are not yet delivering consistently world-class teaching for every student in every classroom in every school.
"Following significant improvements, attainment can be seen to be levelling off, and evidence suggests performance still has a stronger link to socio-economic background than is the case in the world's best systems."
But unions say the link between pupil background and performance has more to do with ministers introducing choice and diversity into schools than teaching quality.
The report says there is scope for strengthening the quality of professional development and the ability to codify and spread best practice in England's schools. Strengths identified include the devolution of resources to schools and three-year budgets, the focus on turning around or closing failing schools, intervening in poorly performing local authorities and reforming teacher training.
Another strength is marketing teaching as a profession.
It describes the newer policies of integrating children's services, allowing good heads to run groups of schools and having a strategy for the whole workforce rather than just teachers as "innovative but not yet proven".
John Bangs, NUT head of education, said: "The reason the effect of social background is playing out in the results is not to do with the quality of teaching.
"It is the result of a rather poisonous cross-party approach to promoting choice and diversity over the last 20 years.
"When you start introducing choice and diversity you start introducing social division. McKinsey has not recognised the elephant in the room."
Martin Ward, deputy general secretary of the Association of School and College Leaders, said: "It is not surprising they pointed to a strong link between pupils' socio-economic backgrounds and performance. This is something we have known about for some time.
"I think we remain a more stratified and hierarchical society than many others and some of the policies of the present and past Governments have not helped in that respect. They have set up systems in the name of choice that by and large have enabled the middle classes to use the system better for their own good.
"For whatever reason historically this has been a country where there has been more sense of social strata and division than with many of the countries that we find ourselves to be lacking against, such as Finland," she added.
The study comes from McKinsey's UK office, the same firm that published a report last year arguing that raising the status and quality of teachers must be at the heart of any attempt to create a world class education system.
The Government picked up on one of the main themes of the document, written by Sir Michael Barber, an adviser to Gordon Brown, by saying it would like all teachers to have masters degrees.
Performance in England's schools has a stronger link to socio-economic background than the world's best education systems.
Academic content and standards are not fully meeting the demands of employers and universities. But most aspects of England's schools system can be rated as 'good' or 'world-class'.
Strengths include the devolution of resources to schools and three-year budgets, the focus on turning around or closing failing schools, intervening in poorly performing local authorities, the reform of teacher training, and marketing teaching as a profession.
There is scope for strengthening the consistency of classroom teaching, quality of professional development and spread best practice.
Newer policies of integrating children's services, allowing good heads to run groups of schools and having a strategy for the whole workforce are "innovative but not yet proven".
The ways and means for helping the disadvantaged
Policy director of The Sutton Trust. It supports educational opportunities for young people from non-privileged backgrounds.
The social class and income gap in education is depressingly persistent. Today's bright children aged three are set to be overtaken by less able but well-off children by age seven. Just 10 per cent of those from the poorest fifth of households go on to gain a degree, compared with almost half of those from the richest fifth.
So what can be done? Investment in high quality early years provision is critical. It is no coincidence that countries which do so well in the social mobility stakes have invested in pre-school education, delivered by a highly-trained workforce.
At school level, we need to even out our socially-selective system which too often serves to entrench disadvantage. This means a focus on fair admissions and the use of more equitable criteria for deciding who gets in to which schools - including our top state and independent schools.
We also need to redouble efforts to get the most talented teachers to our most challenging pupils. And because aspirations and achievement are so intertwined, we cannot ignore the issue of university progression.
It is only through a joined-up approach, which offers particular support at key junctures, that we can hope to break the link between wealth and educational outcomes.
Principal of Capital City Academy, in Willesden, north-west London, and a spokesman for academy heads.
This year, 35 sixth form students at Capital City Academy will progress to university compared with four a year from identical cohorts in the school we replaced.
The expansion of higher education has not significantly narrowed the gap between rich and poor. Around half the population are from the lower social classes, yet only 32 per cent of these reach university.
This is one reason why many academies, like mine, have placed a premium on strong sixth forms as well as establishing the consistent quality of teaching and learning students need.
Academies have twice as many students receiving free school meals as the national average, and are generally located in the poorest areas.
But by improving at twice the national average in numbers gaining five good GCSEs, academies are making a real difference in closing this social divide.
Strong leadership, parental popularity and good links with them are transforming expectations and results.
As the number of academies reaches 400 and more, I believe the gap between rich and poor in our universities will narrow. And with that, I hope we see levels of social mobility that give our poorest young people the chance to make the most of their talents and abilities.
Secretary of Comprehensive Future, the campaign for a 'fair' admissions policy in England.
There are many reasons why lower educational attainment is associated with disadvantage, most having an impact long before compulsory schooling. Nonetheless schools can make a difference. Schools with an academically balanced intake are more likely to raise attainment for disadvantaged children. Government should address the effect of secondary school admission policies or more precisely the effect of selection.
There is evidence of the need to take action. Statistics on school composition showed the level of free school meal segregation in local authorities is more associated with the proportion of pupils in grammar schools than any other characteristic.
The 2006 PISA study found "that early differentiation of students by school is associated with wider than average socio-economic disparities and not with better results overall". So selective systems widen rather than narrow the gap in achievement.
Failing selective entry tests affects a significant proportion of English children. Most of those who fail and have their aspirations lowered will be already disadvantaged.
The Government should take on this challenge instead of leaving it to local people to initiate campaigns which have failure built in to the mechanism.