Nearly one in five children fail to win a place at their preferred secondary school this year, Government figures showed.

Children's Secretary Ed Balls warned that a Government education review had uncovered unlawful admissions practices in a "significant minority" of schools.

Most of the schools involved were faith schools, many of which can legally ask parents for voluntary financial contributions towards their children's education.

But a snapshot survey in three out of 150 local authorities found some schools were "asking parents to commit to making financial contributions as a condition of admission", he said.

In one case, parents were told to complete a standing order at the time they applied for a place.

Mr Balls said such action was "totally unacceptable".

Schools Minister Jim Knight said some of the unlawful activities uncovered in the survey of Northamptonshire, Manchester and Barnet, north London, had been banned under primary legislation. He said: "The fact that there are some things that are singled out in primary legislation that are still going on is shocking."

Ministers have written to 119 schools which have control over their own admissions in these three areas, warning them that they must comply with the new statutory school admissions code which came into force last year.

Mr Balls said 570 primary and secondary schools had been examined in the three areas. He said: "The large majority of schools appear to be complying with the code. However, a significant minority of schools in our sample appear not to be compliant with the code, of which a disproportionate number are voluntary aided or foundation schools."