The colonisation by the middle classes of the best state schools has led to a dramatic widening of the gap in educational performance between rich and poor children in the past year, new figures indicate.

An analysis of government data by the Conservative Party shows that the achievement divide between pupils in the 10 per cent richest and poorest areas of England has grown by more than ten percentage points, compared with fractional increases of less than one percentage point in previous years.

The figures also show that the attainment gap between rich and poor continues to widen as pupils progress through school. At age 7, the performance gap between pupils in the 10 per cent richest and poorest areas was 20 percentage points in 2007. At age 16, however, the gap had more than doubled to 43.1 per cent, suggesting that far from being a leveller, school was increasing the disparity.

The figures underscore the massive influence of parental background on school success. More than 65 per cent of children in the wealthiest group achieved at least five good GCSEs, including English and maths, this summer but the figure for children from the poorest backgrounds was less than 26 per cent.

Michael Gove, the Shadow Education Secretary, said that the system favoured those who were fortunate enough, or rich enough, to live in areas with good schools. “If you have nominal parental choice over school admissions, but an undersupply of good schools, you will find that the sharp-elbowed middle-class parents get access to excellent schools, but those trapped in deprived areas do not,” he said.

Mr Gove said that the dramatic widening of the gap this year, after much smaller incremental increases in previous years, was the result of the cumulative effect of this phenomenon. He noted that pupil performance in the richest areas had improved at twice the rate that it had deteriorated in poor areas.

An additional explanation of the sudden widening of the gap this year may be the influx of immigrants who do not have English as a first language, he suggested.

Conservative plans to allow good new schools to open in deprived areas, with extra cash for children from more deprived homes, would reverse a growing social class gap, he said.

Alan Smithers, Professor of Education at the University of Buckingham, said: “Even where you have good schools in poor areas, like some of the academies, they are progressively taken over by ambitious parents.”

The figures come after recent concern by Christine Gilbert, the Chief Inspector of Schools, that the school system was dividing children along social and economic lines. They show that in 2005 28.2 per cent of pupils in the 10 per cent most deprived areas gained at least five GCSEs, including English and maths, at grades A* to C. In the richest 10 per cent of areas, 56.2 per cent of pupils reached this level, giving an attainment gap of 28 percentage points.

In 2006 the figures were 29.2 and 57.6 per cent respectively, with a performance gap of 28.4 percentage points. In 2007 the figures were 25.3 and 68.4 per cent respectively, with a performance gap of 43.1 percentage points.

A spokeswoman for the Department for Children, Schools and Families said that closing the attainment gap was a priority for the Government. The Government had invested more than £21 billion in childcare and the early years since 1997, so that poor children could get better chances in early life, she said. It was now providing one-to-one tuition and personalised support to help every child to achieve at school, regardless of social background.

She added: “We can only tackle deprivation and poverty by changing the aspirations of young people, their parents and the education system.”

Times recommends

• Students abandon the drink for gyms and juice
• Ofsted: third of Baby P-style reviews inadequate
• Top boys' school picks first woman head