The government has said the change will only affect the wealthiest 500 estates each year but farming unions have said it will apply to thousands of farms.

UFU president William Irvine, who is among farmers from Northern Ireland travelling to London for the rally, said they “see this as the start of a campaign”.

“Where it goes, I can’t predict, but we have a strong mandate from Monday night to follow this through,” he told the Good Morning Ulster programme.

He said the presence of the UK’s four farming unions at the event will show “we have power as an industry”.

About 15,000 farmers have signed the UFU’s petition to overturn the family farm tax, which is to be presented to the secretary of state.

“It will be a wake-up call for the government,” said Mr Irvine.

He added that as well as a meeting with Hillary Benn, “we our demanding more meetings with government to talk about this and get some sense on the table here.”

Since 1984, agriculture property relief has allowed land used for crops or raising animals, as well as farm buildings, cottages and houses, to be exempt from inheritance tax.

From April 2026, however, that will only apply to the first £1m of the estate, with anything more than that value taxed at 20% – half the usual rate.

Research by the Department of Agriculture, Environment and Rural Affairs (Daera) suggests a third of farmers in Northern Ireland will be affected, with the dairy sector particularly badly hit.

Richard Beattie, president of the Young Farmers Club of Ulster, told Good Morning Ulster this new policy could allow external businesses to buy their farmland.

“We’ve all been witness to inflation, and buildings, land and machinery have all been inflated in the last few years, so that £1m tax benchmark is significantly lower.

“This may be seen to help farmers, but the farmer has the least cash to pay the inheritance tax bill, and in some cases other larger businesses might have more cash assets to buy the land.

“Ultimately the farmer will be worse off in this whole situation.”

He added: “Income in agriculture is a very volatile thing and farms are very money-hungry and families with a next generation tend to invest for that next generation.

“The change in this inheritance tax will reduce investment on farms.”