Accountants Kingston Smith have criticised proposals to put a financial value on the contribution of volunteers in charity accounts.
Last month, the Charity Finance Directors' Group released a report, Inputs Matter, which called for a cross-sector standard, such as the minimum wage, to be used to value volunteer hours.
But Nick Brooks, head of the not-for-profit section at Kingston Smith, said the idea could be a mistake.
"Of course organisations want to acknowledge their volunteers' contribution and many already do so by reporting the number of hours contributed in their annual reports," he said.
"But are we not creating more work and expense, as well as valuing volunteer time at a single rate?"
He said that moves to include volunteer hours in the Statement of Financial Accounts could be partly motivated "by a desire to enhance the reported size of the sector".
But Les Jones, deputy chief executive of WWF, who led the team that produced Inputs Matter, defended the inclusion of a financial value for volunteers: "Volunteers are the very essence of many charities and make a huge contribution to the sector," he said.
"This should be acknowledged in money terms in the accounts. A vague footnote hidden away is not good enough. Our motives in making this suggestion had nothing to do with increasing the reported size of the sector."