Victoria Atkins, the shadow environment minister, said: “The OBR itself has said there is great uncertainty about the figures Labor is using to justify its vindictive family farm tax.

“Many farms are asset and cash rich. We’ve heard of farmers facing hundreds of thousands of pounds in inheritance tax, but the income on these farms is around £20,000 a year.

“Labour’s maths simply doesn’t add up. This single policy risks food price inflation, job losses and the end of agriculture as we know it.”

Tax expert, Rachel Griffin, from wealth management firm Quilter, said: “The Government may question whether the controversy surrounding changes to farm property relief is justified, particularly as the OBR predicts these changes will generate £500 million annually.

“At the current rate, total inheritance tax could even exceed OBR forecasts by around £300m this year alone.

“Choosing more targeted restrictions rather than blanket reforms may have been better received.”

In his inaugural Budget on 30 October, the Chancellor cut a number of reliefs from inheritance tax, make pension pots accountable, and reduce exemptions for farms and businesses.