Research from the Department for Education and Skills (DfES), to be published tomorrow, shows that middle-class children have been the chief beneficiaries of record public investment. Results at the worst primary schools have risen rapidly since 1998, and many previously weak schools have caught up with the best.

But while the gap between the best and worst primary schools has narrowed, the gap between children from deprived backgrounds and those from more affluent families has actually widened in the past six years, the research found.

Both sets of 11-year-olds have achieved better results, but middle-class pupils have improved by much more.

The admission by Ms Kelly comes as she prepares to announce that every child in England will receive a free bag of books from the Government to address criticism that too many children leave primary school unable to read adequately. Nine million books will be sent out to children aged from eight months to four years under the £27 million Bookstart programme to encourage parents to read with their children. Titles will include The Very Hungry Caterpillar, Where’s Spot? and We’re Going on a Bear Hunt.

Ms Kelly will use a speech on social mobility to the Institute for Public Policy Research think-tank tomorrow to outline plans for what she calls “a major policy shift” away from targeting deprived schools towards targeting deprived pupils. Speaking to The Times about the new research, she said: “We should be proud of what has happened since 1997. We can say for the first time schools in disadvantaged areas have caught up with schools in more prosperous areas.”

In London, for example, average results are now higher than the national average. She said: “That is a dramatic turnaround. However, this new data shows we have a lot more to do to reach out to those still falling behind.”

Work on a range of new policies is at an early stage, but Ms Kelly said that the “whole class teaching” that formed the basis of the literacy and numeracy drive would be replaced with small group tuition, to help struggling pupils to catch up.

This first research at “pupil level” conducted by the DfES will ring alarm bells across Government. Based on results at Key Stage 2 — tests undertaken by children at 11 just before they leave primary education — it compares the performance of children who qualify for free school meals with the rest of the class. The results of both groups have improved, but the results of children from more affluent families have risen much faster.

Ms Kelly said that she was not shocked by the results, but it was certainly a sharp reminder that there was much more to do on education.

She has been struck by other data showing that social mobility had fallen in Britain since the 1960s. An authoritative study published by the London School of Economics in May showed that children born in 1970 were less likely to break free of their background and fulfil their potential than children born in 1958.

She said: “We now know social mobility declined in the 1970s. It may have increased since 1997. But this is an important issue for a progressive Labour Government. We want every child to have the opportunity to realise their potential and make a contribution to society whatever their background.

“So not only should we care about overall standards rising, but whether children from the most disadvantaged backgrounds share in the rising standards.”

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