A recommendation to abolish a threshold below which expenses and benefits are tax-free would be "an administrative nightmare" for charities with large numbers of volunteers, according to the Charity Tax Group.
Under the existing rules, anyone earning less than £8,500 does not have to pay tax on these benefits.
The CTG said the removal of the limit would be "of considerable concern" because it would mean that any benefit paid to volunteers was potentially taxable.
This would cause major problems for some charities that pay expenses to thousands of volunteers, the CTG said.
"The previous government consulted on the idea of abolishing the £8,500 threshold," said Helen Donoghue, director of the CTG. "Unfortunately, however desirable that proposal may have been for businesses, it would have been an administrative nightmare for charities with lots of regular volunteers.
"We communicated this to HM Revenue & Customs at the time and pointed out that at least one of our members had more than 50,000 volunteers that would need to be covered.
"The proposal was dropped but we will need to make the case again if the idea is resurfacing."