Crucial guidance on how independent schools should meet public benefit tests may not be released by the Charity Commission until this summer - or even later.
From next week, charities whose financial year begins after March 31 will have to detail the public benefit they deliver in their annual reports after laws passed three years ago removed the assumption that fee-charging schools offer a public benefit.
It means independent schools will have to show the commission that they are accessible to pupils from poorer backgrounds in order to retain their charitable status and tax breaks - worth about Pounds 100 million a year.
The commission has now completed its scrutiny of five schools taking part in a pilot study that will provide the independent sector with day-to-day examples of how to meet the guidance.
Some independents have suggested that if they open their facilities to other pupils in their areas or work in partnership with local state-funded schools, this may be as effective a way of proving public benefit as increasing the number of bursaries they offer.
Schools had been expecting to hear the results of the pilot study in the spring, but the commission has admitted it could be June before the results are released.
A spokeswoman said: "We are hoping (to have the results) before the summer, but it could be later."
She said the commission had still not decided how it would announce the results of the pilot, but it is believed schools will be told first in order to be given a chance to correct any errors before the details are released more generally.
Jonathan Cook, general secretary of the Independent Schools' Bursars Association, said the commission should be more open.
"They're the regulator and we don't know when the results will be announced," he said. "We're in their hands, but they're not telling us anything."
The five schools in the pilot scheme - Manchester Grammar (above), Moyles Court School in Hampshire, Pangbourne College in Berkshire, St Anselm's School in Bakewell, Derbyshire, and Highfield Priory School in Preston - have all been visited by the commission and completed questionnaires as part of the process.
Some schools are concerned that any recommendations from the commission to increase the number of pupils on full bursaries would hit their finances just as the recession is deepening.
Simon Northcott, head of the Pounds 5,000-a-year St Anselm's, which has 250 pupils aged 3-13 on roll, said: "If all of a sudden we were told to have 20 more pupils on 100 per cent bursaries, then we would have to find that drop in income somehow.
"The banks won't lend that to us, so we may push fees up, which would end up being more - rather than less - exclusive."
The latest guidance for independent schools was published a week before Christmas. But some said it should have been published before they handed back their questionnaire.
David Williams, head of Highfield Priory, said: "We hope they give us a fair crack of the whip."
In all, a dozen charities - including the five schools - are taking part in the public benefits pilot scheme.
The five independents in the public benefits pilot scheme have been visited by a team of three from the Charity Commission and asked to complete a detailed questionnaire.
The vetting process ended in January, with one or two receiving follow-up calls.
Simon Northcott, head of St Anselm's School in Bakewell, Derbyshire, said he was given just two hours' notice of last October's decision to include his school in the pilot - the unfortunate consequence of the letter being sent to his bursar, who was absent on the Friday when it arrived.
"By the time Monday morning came, we had a two-hour head start on the media," he said.
The Independent Schools Council later criticised the commission for naming the first schools to be scrutinised.
Most of the five have been happy with the way the commission has handled the process, but a number have reported that the questionnaire arrived on school doormats during the half-term holidays last autumn.
But the five remain confident that the Charity Commission is not singling them out for special treatment.
Greg Meakin, head of Moyles Court School in Hampshire, said: "My feeling is that the commission is not seeking to put any undue burden on my school."