The recently proposed tiered structure for probate fees threatens to undermine other improvements made by the government to inheritance tax (IHT), adviser Trystan Lewis has warned.
The Griffin Wealth Management chartered financial planner said the new probate fee structure amounted to just "another form of taxation", which would increase the burden on consumers.
He said: "It's come in at a time when the government have improved the IHT situation with the residence nil-rate band but at the same time they've increased these probate fees.
"So they've given us something on the one hand and then taken something away."
He added: "No one wants to see increased fees. When you take into account IHT, solicitor fees for probate and the new probate fees, it can suddenly mount up significantly which can come as a surprise to clients."
Probate fees will change from the current flat rate fee, where estates worth more than £5,000 pay £215 - or £155 through a solicitor - to a sliding scale based on asset value.
The changes will mean estates valued at below £50,000 are exempt from any charge, but the charge on higher value estates will increase, with fees ranging from £300 to £20,000 depending on the value of the assets before inheritance tax (IHT).
Proposed by Justice Secretary Liz Truss, the new fees are expected to raise about £250m for the Exchequer. The proposal was presented to Parliament on 24 February and will come into effect in May.
However, the Joint Committee on Statutory Instruments issued a warning to the Ministry of Justice in a new report, saying probate fee changes could be illegal without parliamentary approval due to their resemblance to a tax.
It said: "It might be acting beyond the enabling powers [of the Ministry]…because [Truss] would, in substance, be imposing a tax on estates rather than prescribing probate fees."
Old Mutual Wealth financial planning expert Rachael Griffin said it was unsurprising the government had come under pressure about the changes.
She said: "When the plans were announced the industry raised concerns that the fee hike for some would be extortionate.
"A system where over 90% pay less than £1,000 would be a positive move. However, it adds more uncertainty to estate planning as more changes could be coming down the pipeline."
Residence nil-rate band
The residence nil-rate band came into force on 3 April and added an additional £100,000 IHT allowance to the current nil-rate band of £325,000 for family homes. The extra allowance is due to rise to £175,000 by 2020, which would mean by that point a couple could pass a £1m home to beneficiaries free of IHT.
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