The government's flagship free schools programme is unlikely to boost access to good schools as they are too expensive, research has suggested.
The Bristol research said it was "inconceivable" more than one parent-founded school would be set up in an area with spare places.
Their impact on raising standards in poor schools was a "one-shot game" and would be minimal therefore, it said.
But the government insisted its reforms would raise standards.
Published in Research in Public Policy and carried out by a team from Bristol's Centre for Market and Public Organisation, the research examines the likely impact of government reforms.
It said free schools were potentially an important part of "open public services" and are highlighted in the government's schools reform plans as a key route to achieving fairer access.
It said: "Free schools do offer this in principle: parents very dissatisfied with their state school can opt out and set up their own school.
"But there are two reasons why free schools are unlikely to be the best answer to this. First, there are very significant set-up costs, both in time and energy from the founders, but also in the straightforward sense of acquiring premises.
"While currently these are being generously funded by the government, this cannot continue if the policy matures and spreads."
In November, the government said it would spend an extra £600m building 100 free schools over the next three years.
The research added: "It seems inconceivable that any local area with one free school and plenty of spare school capacity would be offered the resources for many others.
"So as a performance discipline device, this is a one-shot game, not a process of continuing pressure on low performing schools, which is what is needed."
Spare places are often in areas where schools perform badly because many parents will have opted out of sending their children to local schools.
The research claims the main way of tackling fair access would be to end the policy of allocating places to pupils on the basis of their proximity to the school.
It added: "Our research shows that this proximity rule strongly favours children from more affluent family backgrounds.
"The gap in accessible school quality between rich and poor families widens by over 50% once a proximity criterion is imposed."
The researchers also claimed that two decades of parental choice and school competition, through the school performance tables, had done little to improve quality.
A spokesman for the Department for Education said: "Parents rightly want to send their child to a good local school, with high standards and strong discipline.
"Free schools have the autonomy to make decisions that are right for local children. Evidence from around the world is clear that giving teachers and heads more freedom in the classroom helps to raise the quality of education."
"Our reforms will drive up standards right across the board - bring the best graduates into the classroom, giving new powers for teachers to keep order and developing a world-class curriculum."