Plans to allow new faith schools in England to increase the share of pupils they take on religious grounds will not improve standards, a report says.
The Education Policy Institute research body also said the move was unlikely to boost social mobility.
The proposal is part of a range of measures, including opening new grammar schools, aimed at boosting the number of places in high-performing schools.
The government said faith schools were some of the best and most popular.
And the Church of England, which is the biggest provider of faith schools in England, said its 4,700 schools offered “a distinctive blend of wisdom, hope, community and dignity”.
The Department for Education’s plans to allow new faith schools to recruit more than half of their pupils on religious grounds are based on the assumption that children do better in these schools.
They appeared in the Green Paper, Schools that Work for Everyone, which sets out plans to allow successful schools to expand.
At the moment new faith schools, set up as free schools, can recruit only 50% of pupils on the basis of faith.
But existing faith schools have no limits on the percentage of pupils they can recruit on religious grounds, although some Church of England schools admit quotas of non-religious pupils.
The more oversubscribed a school is, the more likely it is to have higher numbers of pupils admitted on religious grounds.
This can mean families of pupils are required to attend church, synagogues or mosques on a weekly basis.
The report finds that while pupils in primary and secondary faith schools, including disadvantaged ones, do get better results, this does not take account of the inherently bright nature of these pupils.
Analysis in the report, which looked at the results of pupils at all schools in England, found faith schools took a lower proportion of the poorest pupils.
It also found such schools had a higher proportion of children with high prior attainment – those scoring highly in assessments and tests in the early years of school.
The report said that once this high prior attainment had been taken into account, faith schools performed little or no better than non-faith schools at primary level.
At secondary level, pupils recorded small average gains of just one-seventh of a grade higher in each of eight GCSE subjects.
The report said: “We found that at both primary and secondary level, faith schools tend to admit fewer pupils from disadvantaged backgrounds, fewer pupils with special educational needs and more pupils with high prior attainment than the national average.
“In terms of pupils from disadvantaged backgrounds, faith schools were less representative of their local area than average at both primary and secondary level.”
It argued that faith schools were on average slightly more socially selective than high-performing schools, but at secondary level much less socially selective than grammar schools.
It concluded that: “While encouraging more faith schools to open may help the government to meet its requirements to provide sufficient school places, the proposed policy is unlikely to yield school places that are of a significantly higher quality than that offered by non-faith schools.”
But the Department for Education said it wanted to open up good schools to all children, irrespective of background.
“Faith schools are a vital part of this – they are among the best schools in the country and places are in high demand.
“That’s why we want to remove the ineffective faith cap to establish even more good schools, while introducing new measures to improve inclusivity and diversity.”
Rabbi Dr Jonathan Romain, chairman of the Accord Coalition, which opposes religious selection in schools, said: “Opening new faith schools that can religiously select pupils will undermine community cohesion, harm the life chances of children from deprived backgrounds and not raise overall standards.”
But the Church of England, which says more than half of its schools have no faith-based admissions, said: “We have no plans to change our approach to faith-based admissions criteria following the government’s proposals to relax the 50% rule.
“In fact, over half our schools have no faith-based criteria at all.
“Our schools are not faith schools for the faithful, but Church schools for the community.”