Lawyers have urged the Charity Commission to publish guidance on how the Equality Act will affect charities before the legislation comes into force.
The act, which will take effect on 1 October and be enforced by the Equalities and Human Rights Commission, means that charities will no longer be able to restrict the services they provide to certain beneficiary groups solely on the grounds that the restriction is contained in their governing documents.
Instead, charities will have to show that the restriction is "a proportionate means of achieving a legitimate aim", or is intended to prevent or compensate for a disadvantage linked to a certain characteristic, such as age, disability, race or sexual orientation.
Ian Davies, a senior consultant at law firm Wilsons, said charities were unsure about how strictly the legislation would be enforced, and how a "legitimate aim" would be defined.
"There are a large number of charitable funds that, for instance, provide grants for the education of girls only," he said. "They are concerned about whether the EHRC will be satisfied that they have passed the test."
Davies said trustees concerned that their charities might fall foul of the new legislation would be in "double jeopardy" once the law came into force. "If they continue to apply the restrictions they may find they are in breach of the Equality Act. But the alternatives are to either stop using their funds, or to plough on regardless of their concerns, which means they would be in breach of trust," he said.
"It is outrageous for a regulator to leave charities in this sort of limbo this close to an implementation date."
Nicola Evans, a senior associate at law firm Bircham Dyson Bell, said she hoped a member of staff at the commission would provide guidance on the legal position of voluntary organisations at the Charity Law Association's conference in October.
A commission spokeswoman said it would produce guidance for charities later this year, but that it was not clear whether this would be before or after the law came into force.
"Provided that charities are satisfied that there is disadvantage or need affecting the group they limit their services to, they will continue to be within the scope of the part of the new law known as the charities exemption," she said.