According to the briefing, published on 21 May, while the proportion of pupils from state schools (as opposed to independent ones) attending Oxbridge has slowly crept up, absolute numbers peaked in 2002 in Oxford and 2008 in Cambridge.

The report, which looks at entrance to the prestigious universities throughout the 20th century, cites a study published in 1938-39 as the first comprehensive survey of entrants' school backgrounds. It found that 24.2 per cent of home entrants to Oxford and 19.3 per cent to Cambridge started their education in public elementary schools. The equivalent for all other universities was 54.8 per cent.

A survey carried out in 1961 found that 34 per cent of students at Oxford and per cent at Cambridge had attended state secondary schools, compared with 63 per cent across the sector.

The briefing shows that by 2011, the figures for both universities stood at about 56 per cent.

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Oxford's access agreement with the Office for Fair Access does not include a simple state-school target, but Cambridge's agreement aims to increase the proportion of UK state entrants to 62-63 per cent by 2016.

Oxbridge colleges also vary greatly in the number of students they recruit from non-fee-paying schools. At Oxford, between 2009 and 2011 this varied between 45 per cent at Trinity College and 70 per cent at Mansfield College.

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At Cambridge, in 2011 Trinity College had the lowest share of state-school students (46 per cent), while Peterhouse had the highest (74 per cent.)