The gap between poorer and richer children's results in primary schools has grown, research by the Department for Education and Skills indicates.
But primary schools with deprived intakes have made the fastest improvements.
The findings are significant because they have helped persuade Ruth Kelly, Education Secretary, that the Government needs to shift its focus from deprived schools to deprived pupils.
"We must treat seriously the possibility that - despite all our efforts - who your parents are still affects attainment as much in 2004 as it did in 1998," she said. "We need to complement our focus on the success of institutions with one that really looks hard at how every child is doing."
The analysis indicated that schools with the highest numbers of pupils eligible for free meals saw the biggest increases between 1998 and 2004 in students passing key stage tests at 11 at level 4 or above. Passes in English tests at these schools rose by 18 percentage points compared to 10 per cent in the schools with the least deprived intakes.
The change in schools' average point scores was less striking, but still suggested the social gap had narrowed.
However, the DfES found different results when they looked at how individual children had performed.
Pupils eligible for free school meals made fractionally less of an improvement in their point scores in English and maths than others.
Ms Kelly said that the findings added weight to the Government's drive to personalise teaching, which would be a central part of its white paper on education due later this year.