The Institute of Legacy Management has urged the government to let MPs vote on proposed changes to probate fees that could cost charities £10m a year.
A report published with the Chancellor's Spring Statement this week indicated that the new fee system was a tax.
Ministers have been careful to describe the change as an enhanced fee rather than a tax because the latter would require a commons debate and a vote to become law, experts said.
By classing it as a fee, the government is able to implement changes by using a statutory instrument, which is a secondary form of legislation that does not require a vote.
The Non-Contentious Probate (Fees) Order, which was passed last month, will remove the existing flat-rate probate fee of £215 and replace it with fee bands.
Once the statutory order is laid before parliament, it could become law in 21 days.
Estates of more than £50,000 will then pay between £250 and £6,000, with the maximum amount reserved for estates worth more than £2m.
The ILM has estimated this could cost charities £10m a year in legacy income.
The Office for Budget Responsibility's latest Economic and Fiscal Outlook for March 2019 says: "The new probate fee structure is expected to generate £155m a year in additional tax receipts."
The report adds that the Treasury expects the Office for National Statistics to classify the fee as a tax in the national accounts.
This appears to contradict Lucy Frazer, the minister responsible for legal fees, who told parliament last month: "What we are proposing is an enhanced fee, not a tax."
Matthew Lagden, chief executive of the ILM, said it had always regarded the changes as a tax "because the new charges will be significantly higher than the costs of delivering the service".
Lagden said he hoped the OBR report's use of the word "tax" would "cause the government to take stock and either reconsider or bring forward legislation so it can be debated properly".
He added: "We understand that the House of Lords shared this view, which is why they felt unable to vote it down because by convention they do not vote down taxation measures."
The ILM was one of four sector organisations that wrote to Frazer this week suggesting a reduced or discounted rate on probate fees for estates that include legacy gifts.
The Institute of Fundraising, another signatory, also believes the change is in effect a tax.
A spokeswoman said: "We therefore continue to urge the Ministry of Justice to review this for a fairer cost that is proportionate to the work involved and includes a charitable exemption."
However, the Ministry of Justice maintains that, even if the ONS classifies the change as a tax, it still considers it a fee.
An MoJ spokesman said: "This is not a tax and any decision by the ONS to define it as such would be purely for accounting purposes.
"The income raised from probate fees will go towards funding a more efficient and effective courts and tribunals system."
Nicola Evans, charities counsel at the law firm BDB Pitmans, who described the new system as a stealth tax last year, as did a committee of peers, said: "The government must always have known that its proposed ‘fee’ would be classified as a tax, just as its previous proposal was in 2017.
"It is not very encouraging that the Ministry of Justice seeks to split hairs in this way and persists in its denial that the government is trying to push through a new inheritance tax by the back door, with none of the exemptions that apply to the existing inheritance tax."