Michael Gove, the education secretary, will announce that he plans to give schools the right to give preference in admitting children from poor backgrounds.
The freedom, likely to prove controversial with wealthier parents, will be available for any primary or secondary school in England and Wales. It is designed to encourage free schools to be set up in poorer areas and will be introduced with a pupil premium, a subsidy for children on free school meals.
Details of the size and funding of the premium, a cherished policy of the Liberal Democrats, are due to be announced with the spending review in October.
Many studies have suggested that middle class parents are willing to increase the odds of their child getting into a top school by moving house, attending church or hiring private tutors, while poorer parents find the admissions system too complex.
Gove said : "What we wanted to do is see how we could give priority in admissions to children from poorer homes. If there are academy sponsors who especially want to target poorer children... then we're allowing those schools to say, 'we would like to give preference to a set number of children eligible for free school meals'.''
Asked if many schools would want to take up this freedom, Gove said: "It may well be that there are schools that say 'we want to go out of our way to attract poorer children.' Historically we haven't achieved as well as we should, particularly given the nature of Britain's stratified and segregated education system".
He said the proposal would dovetail with the pupil premium since "schools would know that the more children they managed to attract from poorer backgrounds, the more cash they would be able to have".
He claimed: "Schools would go out to parents who may well have thought in the past, that they have got no chance of getting in there."
Gove is also planning to extend the definition of poorer pupils to include all children who have ever been eligible for free school meals, not just those eligible now, to avoid excluding households where one parent has just got a good job.
The changes would be introduced in 2012 after consultation in the autumn. Gove is still negotiating with the Treasury on the size of the pupil premium.
He has frequently cited figures showing how poorly children on free school meals do in comparison with children at independent private schools. Half of pupils entitled to free school meals are concentrated in a quarter of secondary schools, while the top secondaries take on average 5% of pupils entitled to them.
The shadow children's secretary, Ed Balls, said: "After weeks of reports about how the poorest will bear the brunt of the coalition's cuts, it's no surprise that the government is rushing out an announcement that looks superficially like it might help the most disadvantaged.
"But simply changing the admissions code will do nothing to counter-balance the coalition's cuts to free school meals, new school buildings and catch-up programmes."