The education system's failure to address underachievement in poor children is an "appalling injustice", the Ofsted chief inspector has warned.
Sir Michael Wilshaw said it saddened him that disadvantaged pupils are let down and that schooling is still "mediocre", during a speech at Wellington College, Berkshire, as part of the Festival of Education.
He said that while the lot of disadvantaged children in primary schools had improved, the attainment gap between children on free school meals (FSM) and their wealthier peers in secondary education had not changed in a decade.
"Despite all the good intentions, the fine words and some imaginative initiatives, we are not making a real difference.
"The needle has barely moved," he said.
Sir Michael told the audience that in 2005 the attainment gap between FSM and non-FSM pupils in secondary schools was 28%. The figure has not changed in 10 years.
He said: "Our failure to improve significantly the educational chances of the poor disfigures our school system. It scars our other achievements. It stands as a reproach to us all."
The Ofsted chief said the "irony" of speaking at one of the country's top independent schools about the challenges poor pupils face was not lost on him.
"I wonder how many people realise just how badly the poorest pupils have been let down in some of the wealthiest parts of the country?" Sir Michael asked.
He described the attainment gap between pupils in secondary schools as an "appalling injustice" and an "inexcusable waste of potential".
"The attainment gap between FSM and non-FSM secondary school children in West Berkshire is 31 percentage points. In Kent it's 34. In Surrey it's 36. In Buckinghamshire it's 39. And, in Reading, it's a whopping 40 percentage points - all far in excess of the national gap of 28," he said.
Sir Michael, who is coming to the end of his tenure as chief inspector, blamed the attainment gap on the "crossfire" between left and right wing politics; those who have argued that children don't need "structure" in school; a failure to develop a curriculum pathway into apprenticeships; poor teaching; and an inability to deliver strong leadership where necessary.
A Department for Education spokesman said: "Every child, no matter their background, deserves a world-class education. Thanks to our reforms there are 1.4 million more children in good or outstanding schools since 2010.
"But we know there is more we can do. The pupil premium - worth £2.5bn this year - helps disadvantaged pupils reach their potential and we have overhauled the national curriculum so all pupils have access to the world-class education they deserve.
"In our recent white paper we set out plans to tackle areas of under performance - to ensure no child is disadvantaged just because of where they live. Furthermore, we are ensuring that all schools have the resources they need through the introduction of a new National Funding Formula that, for the first time, will make sure funding is genuinely matched to need."