A "scandalous" number of schools in England are demanding financial contributions from parents that are potentially illegal, research suggests.
The British Humanist Association (BHA) found that, of the first 100 schools it identified as asking for payments, 89 were faith schools.
As things stand, schools are allowed to seek voluntary donations from parents, but must not pressurise them and must make it clear that there is no obligation to pay up.
They are also not permitted to accept any kind of payment in relation to admissions.
A Department for Education spokesman said: "The School Admissions Code is clear that schools must not request financial contributions - whether voluntary or compulsory - as any part of the admissions process, including when offering a place.
"Any claim that the School Admissions Code has been breached will be investigated."
The BHA said it found a large number of schools asking parents for money whilst either putting undue pressure on them to contribute or not making it clear that contributions were voluntary.
As an example it highlighted one Church of England primary school asking parents for an annual payment of £30 for its "building fund/capitation for parents".
Its website states: "This is not a voluntary contribution but it is a payment all Church of England schools require to maintain the school buildings and classrooms."
A Catholic primary school's website asks parents for £100 per family towards "building works", saying: "As a voluntary aided school, parents of the pupils attending the school are responsible for contributing 10% towards all building works."
The BHA claims that a number of schools also stressed that the requested contribution, far from being voluntary, was a minimum amount, encouraging families that could afford to pay more to do so.
One school was even found to have suggested that parents should contribute to the fund by using the money they were saving as a result of receiving free school meals.
Commenting on the findings, BHA chief executive Andrew Copson said: "It is scandalous that the 10% of capital costs which religious groups running state schools are obliged to pay is actually being demanded unlawfully from parents and even that money is being demanded at all.
"The fact that these abuses occur so widely, and that it is only down to our investigation that they have been exposed, should call into question the whole system of regulation surrounding state-funded faith schools."