The Charity Commission's guidance on public benefit by fee-charging schools is "legally erroneous in important respects", the Independent Schools Council will argue at the Royal Courts of Justice today.
It will also argue that the commission has made it difficult in practice for private schools to know whether they have done enough to meet the requirement in charity law that they provide a public benefit.
The ISC's arguments are outlined in a submission published on the charity tribunal's website to coincide with today's opening of a judicial review of the guidance in the Upper Tribunal (Tax and Chancery).
The tribunal is simultaneously considering a request from the Attorney-General for clarification of how charity law applies to fee-charging schools.
The ISC submission says: "The guidance which the commission has issued contains important errors of law which lead to very significant practical consequences."
It says these errors include the requirement that those unable to pay the fees charged by an independent school should nonetheless be able to benefit materially from its activities in a way that is related to the school's charitable aims.
It goes on: "The commission appears to be adopting an approach, which in substance makes it mandatory for independent schools to provide free places funded by bursaries equivalent to what the commission judges to be a sufficient proportion of the school's income, although without any clear indication of exactly what the commission will regard as sufficient."
A submission from the charity commission shows it will stand by its guidance: "If trustees administer a charity for the advancement of education in such a way that benefits are available only to persons able to pay fees which a substantial part of the public (including the poor) cannot afford, they are not administering it to further the purpose for which it was established, and they may thereby be in breach of duty."
The case is scheduled to end on 27 May.