For most families, inheritance tax is paid at a 40pc rate on anything above £325,000. There are a number of exemptions such as spouses being able to transfer assets tax-free.
The £325,000 threshold, which has been in place since April 2009, will be frozen until 2029-2030.
From April 2026, farms worth more than £1m will be eligible for inheritance tax at a reduced rate of 20pc.
The Chancellor’s reforms have drawn ire from farmers, with more than 20,000 farmers and high-profile supporters, including Jeremy Clarkson and Lord Lloyd-Webber, descending on Westminster to protest against the policy.
Mrs Reeves also halved the relief of shares in the alternative investment market (AIM), meaning that 20pc will be paid on shares which have been held for two years.
The OBR report forecasts 308,000 estates would owe inheritance tax to HM Revenue and Customs (HMRC) between now and 2030.
The spending watchdog estimated that an additional £2.5bn will be raised by the inheritance tax changes by 2030. Some £500m would be raised by changes for businesses and farms, and £1.5bn by levelling death duties on pensions.
Receipts for inheritance tax fluctuate throughout the year, as there are more deaths in the winter than in summer.
Alex Davies, of Wealth Club, said: “Inheritance tax was already an absolute cash cow for the Government.
“The extreme changes announced in last month’s Budget which badly affect farmers, business owners, pension policyholders and investors, mean these figures are only going to increase over the coming years.”