Present regulation allows farmers' heirs to escape the tax, which raises £2.5bn a year to the Exchequer, but is only available to legitimate farmhouses.

The lobbyists claim they lose the break when they change the nature of their businesses, as encouraged to do by the government.

The Inland Revenue claims the relief should only be available for farmers, as similar tax breaks are available to other businesses.

But the Country Land and Business Association, which is lobbying for the change, wants the government to alter the tax break so farmers heirs are not faced with a huge IHT burden, reports the Financial Times.