The OSCR decided in July that the High School of Dundee did pass the public benefit test set out in the Charities and Trustee Investment (Scotland) Act 2005.
But Third Sector has learned that a paper circulating in the offices of the Scottish Council for Voluntary Organisations is heavily critical of the arguments used to justify the regulator's decision. The paper accuses the regulator of negligence.
The OSCR had to decide whether the school's fees of up to £8,304 a year meant that public access to its services was "unduly restrictive". The regulator compared the fees to the cost of state schooling and considered the school's bursaries.
But the paper - written by Stephen Maxwell, associate director of the SCVO and a member of the OSCR's advisory group for its 2006 Meeting the Charity Test guidance - says the regulator should have compared the fees with average household disposable income in Scotland, which is £16,272.
"Judging by its report, the OSCR made no attempt to determine what proportion of the local population might reasonably afford to pay between £5,000 and £8,000 per year per child," the paper says. It also criticises the OSCR's approach to deciding whether the High School of Dundee causes any disbenefit to the public.
The paper is likely to be sent to the OSCR and MSPs. The regulator has said the Dundee decision does not establish definitive benchmarks for the independent school sector.