The UK tax office took in £379m, far above the £260m monthly average for the last 10 years.
Additionally, in the three months to the end of May HMRC took in £1bn of inheritance tax payments, the highest three month amount since 2007.
Private client law firm Wilsons said the record sums reflect an “urgent need” to reduce the tax burden on the estates of ordinary taxpayers.
It added that the Treasury’s proposals to reform inheritance tax by raising the tax free amount applied to the value of some estates to £1m will add complexity to the IHT system, and could lead to distortions in the housing market.
The proposed relief would only apply to a main residence being transferred to children or grandchildren by a married couple or civil partners. For all other taxpayers, the tax free amount would remain at £325,000, the level it has been since 2009.
Belinda Watson, a partner at Wilsons, said: “Efforts to reduce the burden of IHT on taxpayers are to be welcomed but these proposals are far from unalloyed good news and add unnecessary complexity to the system. It would be better and fairer simply to raise the nil rate band for all estates.
“Current reform proposals are particularly targeted at families whose homes have increased in value but leave some big problems unanswered. If you do not have your wealth stored up in the family home you do not benefit from this reform.
“There is also a worry that the cost of allowing this more generous relief to certain homeowners may start to put pressure on existing more targeted but, hugely important inheritance tax reliefs.”
She added the last time IHT take was at such a high level was when the law was changed to allow married couples to share their tax free IHT allowances more easily.
Graph showing revenue generated by inheritance tax since May 2005