The Coalition for a Charities Act has written to Clarke to say that Charity Commission guidance - which will be used to assess a charity's public benefit if and when the Charities Bill becomes law - is not strong enough on the subject of fee-charging organisations.
According to the Charity Commission's interpretation of case law, such charities will only have to prove that the less fortunate are "not entirely excluded".
"We do not believe this goes far enough," the coalition wrote. "Anyone able to benefit from a charity's services must have a reasonable chance of doing so."
The coalition insists it is not waging a war against public schools in particular but argues that the Charity Commission's guidelines, which the Government has deemed sufficient to determine public benefit, are not challenging enough on the question of public access in relation to fee-charging charities.
"It is our strong view that the relationship between public benefit and public access to services still needs to be resolved," wrote the coalition, which includes in its number the NCVO, the British Red Cross and Help the Aged.
The group, which also includes Leonard Cheshire, Citizens Advice and the RNID, has requested that the Bill be amended so that public access is given greater consideration in assessing public benefit.
"We're not saying charities shouldn't charge fees," said Belinda Pratten, senior policy officer at the NCVO. "One of the things the Charity Commission should take into account when assessing public benefit is the extent to which potential beneficiaries are excluded because of fees."
The move comes after the news that Labour MPs intend to challenge the charitable status of public schools when the Bill is debated in the Commons.
They plan to resurrect an amendment to compel the Charity Commission to consider the "impact on access to charitable benefit of fees charged".
- See Letters, page 25.