Almost a fifth of Scotland's private schools faced the threat of losing their charitable status unless they widened access and improved public benefit.

The Office of the Scottish Charity Regulator (OSCR) has reviewed 52 private schools since 2007 and failed ten largely on the grounds that their means-tested assistance was "insufficient" and their provision of fee-free activities "minimal".

All subsequently widened access and addressed other failings but OSCR said it will be on the lookout for "backsliding" in a new report on fee-charging schools.

Hutchesons Educational Trust, Lomond, Merchiston Castle, St Leonards, Fettes College, St Columba's, St George's School for Girls, Wellington and Loretto all failed due to restrictive fees.

Non-boarding fees in Scotland's private schools ranged from £1,500 to £11,237 per year while boarding fees ranged from £15,450 to 25,860 per year.

Around a tenth of all private-school pupils had a bursary but in some schools it was little more than one in twenty. Bursaries ranged from a tenth of the fee right up to full payment.

The median income spent on bursaries across the board was 6.1%, with some spending as little as 4.6% while some small specialist schools spent up to 42.1%.

Jordanhill School failed because its constitution made it subject to control by Scottish ministers and their influence was subsequently written out.

A review of Fernhill School was suspended as the school encountered financial difficulties while Struthers Memorial Church, of which Cedars School of Excellence is a part, is currently subject to a separate OSCR inquiry.

OSCR could not find any evidence to support the views of some sections of society that private schools have a negative impact on local state schools and "have a divisive influence in society".

But its ongoing reviews were driven by concerns "that there remained a substantial risk that fee-charging school charities would not comply with the public benefit requirements of the charity test".

OSCR said: "A high proportion (10 out of 50) of the high-fee-charging school charities we looked at failed the charity test when we first reviewed them, almost all on the basis that the fees they charged unduly restricted access to the educational benefit they provided.

"This indicates that the initial perception of the high risk of failure presented by these charities was correct."

It added: "Our future regulation of these charities needs to reflect what the continuing likelihood is that any one of these charities will fail to provide public benefit.

"It is therefore reasonable for OSCR to continue to maintain a higher level of vigilance concerning the compliance of this group of charities.

"On the other hand, it is clear that there is a better awareness among the charities reviewed of what is required to pass the charity test.

"Any future failures would represent backsliding from a position of now having met the public benefit requirements, rather than of never having met them.

"On the basis of the reviews and the action we have taken we are now in a position where all of the schools reviewed meet the requirements of the charity test."

Conservative young people spokeswoman Liz Smith, a former pupil and teacher at the fee-paying George Watson's College in Edinburgh, said: "This report demonstrates the effectiveness of the 2005 charities test legislation which was unanimously agreed by all parties at Holyrood.

"That legislation rightly ensures that there is a very rigorous test for all independent schools to undertake prior to being awarded charitable status.

"The report makes it very clear that the independent schools have been more heavily scrutinised than any other group within Scotland's 22,000 charities and that any concerns about individual schools have been fully addressed prior to the award of charitable status.

"If schools do not meet these criteria, they fail the test.

"That is exactly as it should be and should put an end to those who argue that all independent schools should be stripped of charitable status."