The Ministry of Justice has said it will go ahead with planned increases of up to £20,000 in probate charges despite a warning by a panel of parliamentary experts that they could be illegal.
The parliamentary Joint Committee on Statutory Instruments said Lord Chancellor Liz Truss may have overstepped her powers in ordering the increases which had the "hallmarks of taxes".
In a highly critical report, the committee drew the "special attention" of both the Houses of Parliament to the proposed changes.
However the MoJ said it would go ahead with plans to present them to Parliament with a view to implementing them "as soon as possible".
Under the Government's proposals the current flat rate fee of £155 or £215 will be replaced by a sliding scale, depending on the value of the estate, rising to £20,000 for those worth more than £2 million.
The change is designed to raise £300 million to fund the courts and tribunal service.
The committee, however, questioned whether Ms Truss was entitled to order such a large increase - using the provisions of the Anti-social Behaviour, Crime and Policing Act 2014 - without the clear approval of Parliament.
"The charges ... appear to the committee to have the hallmarks of taxes rather than fees, particularly in view of the amounts that would payable for larger estates and the scale of the proposed increases (from £155 to as much as £20,000 - a rise of nearly 13,000%) - and because the charges are disproportionate to the service provided by the Probate Registry," it said.
"It is an important constitutional principle that there is no taxation without the consent of Parliament, which must be embodied in statute and expressed in clear terms.
"The committee is doubtful whether section 180 of the 2014 Act does in express words entrust the Lord Chancellor with the power to impose charges of the magnitude proposed by the draft order."
The findings are a further blow for Ms Truss, who been criticised by the judiciary for failing to defend judges attacked over the Article 50 court ruling and wrongly announcing that reforms sparing alleged rape victims cross-examination were being introduced nationally.
A MoJ spokesman said: "Our plans to introduce new probate fees remain unchanged. We will introduce a fairer system, meaning over half of estates pay nothing and over 90% pay less than £1,000.
"They will be considered in Parliament after Easter, and come into force as soon as possible."