A shake-up of probate charges is being brought in as a "back-door" tax hike on bereaved families, it has been claimed.
Insurer Royal London said the Ministry of Justice (MoJ) was using the changes to raise revenue from people at their time of greatest need.
Grants of probate currently cost a flat fee of £155 when applied for by a solicitor and £215 when done by an individual.
Under the new system, estates worth more than £50,000 will attract a sliding-scale fee ranging from £300 to £20,000.
Royal London said a freedom of information request it made to the MoJ showed that the ministry could not provide the cost of handling applications broken down by the size of an estate.
The ministry said it did not have such information "because there is no legal or business requirement" to do so.
Royal London's director of policy Steve Webb said this showed the move was about raising revenue, not covering costs.
"The Government is treating bereaved families as if they were a 'nice little earner'.
"It is one thing to make a reasonable charge for the provision of a public service. But the Ministry of Justice has now admitted it does not know the unit cost of handling a probate application and sees no reason to find out what it is.
"This is clear evidence that the new charging structures are nothing to do with recovering the reasonable cost of processing probate applications and are simply a back-door way of raising money from people in their time of greatest need.
"The Government should think again before going ahead with this tax hike on bereaved families."
The changes will see the upper limit of the value of estates that attract no fee rise from £5,000 to £50,000.