The Social Mobility and Child Poverty Commission study of more than 4,000 business, political, media and public sector leaders claim that small elites educated at posh independent schools and Oxford or Cambridge university still dominate top roles, creating an almost "closed shop at the top".
The UK government-backed commission's 76-page report mostly focuses on analysis, but it does include recommendations, saying government, schools, universities, employers and even parents all need to play their part in promoting social diversity.
"Locking out a diversity of talents and experiences makes Britain's leading institutions less informed, less representative and ultimately less credible than they should be," warns commission chairman Alan Milburn in his foreword to the report.
"This risks narrowing the conduct of public life to a small few who are very familiar with each other but far less familiar with the day-to-day challenges facing ordinary people in the country. That is not a recipe for a healthy democratic society," he adds.
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The commission says its findings are based on one of the most detailed analyses of its type ever undertaken.
But 71 per cent of senior judges, 62 per cent of senior officers in the armed forces, 55 per cent of permanent secretaries in Whitehall, 53 per cent of senior diplomats, 50 per cent of members of the House of Lords and 45 per cent of public body chairs did so.
Oxbridge graduates were found to have a stranglehold on top jobs.
The report says the judiciary is the most privileged professional group with about 14 per cent of judges having attended one of just five leading independent schools - Eton, Westminster, Radley, Charterhouse and St Paul's Boys.
It calls for a national effort to "break open" Britain's elite with employers publishing data on the social background of staff, university-blind job applications and non-graduate entry routes, the government tackling unpaid internships that disadvantage those too poor to work for nothing, and senior public sector jobs being opened up to a wider range of people.