The government's inheritance tax changes will affect more than 75% of English and Scottish farms of 50 hectares in size or more, new AHDB analysis warns.
It shows that 42,204 out of 54,938 farms – around 76.8% – will be impacted by the controversial changes, which were first unveiled in the budget.
That is 33,286 farms out of 41,602 (80%) for England and 8,918 farms out of 13,336 for Scotland (67%).
The new IHT rules will see the full 100% relief from inheritance tax restricted to the first £1m of combined agricultural and business property, from April 2026.
AHDB's analysis looked at average balance sheet data mainly sourced from Defra, the Farm Business Survey and the Scottish government.
More than half affected are involved in cereals or general cropping production as their main enterprise, with the rest predominantly livestock producers or mixed farming operations.
When calculating total assets of farm holdings below 50ha, AHDB assumed that average farm values will be less than £1.325m and will therefore not fall into a category that is at risk of being affected.
AHDB analyst Tom Spencer said: “Our calculations show that cereals and general cropping farms are the most likely to be affected due to their scale and asset size.
"For livestock farms, it is those businesses with single person ownership that are most at risk.”
AHDB has already reported that due to the low rate of return on net current assets in farming, the most cost-effective way a cereals producer could pay their expected tax burden would be to sell parcels of land.
It comes as the NFU consulted with former Treasury and Office for Budget Responsibility (OBR) economists to inform its analysis of the impact of the IHT changes on farms.
This concluded that up to 75% of working farms could be affected, with some facing tax bills running into hundreds of thousands of pounds.
And last week, the OBR produced a new report saying that government changes to IHT for farmers would likely leave elderly farmers exposed, with no time to manage their way through the new policy.
AHDB's economics director, David Eudall said it was now critical for any affected farm to seek out expert tax and business planning advice.
"Succession planning was already important in agricultural farming businesses, now it is essential," he explained.
"There are 300 working days until 1 April 2026, when the tax changes come into effect. This means 140 farms across England and Scotland per working day, from today (28 January) will need to ensure their business is set up to manage their tax implications."
Next month, MPs are set for a major debate over the impact of the tax, after a petition on parliament.uk's website reached over 100,000 signatures.
The House of Commons petitions committee agreed that a debate on the subject will take place on 10 February at 4:30pm.
Campaigners at Save British Farming and other industry groups are planning a new rally outside Whitehall on the same date.
