Sir James has skewered Chancellor ' Budget, warning it will be the "death of entrepreneurship".
Writing in , the billionaire inventor and founder of the Dyson company, has accused the Chancellor of "spiteful" after , family businesses and multi-million pound estates, as well as raising taxes on school fees.
One of Britain's leading businessmen, Mr Dyson wrote that Reeves' changes were an "ignorant swipe at aspiration".
He writes: "Rachel Reeves is killing off established family businesses, and any incentive to start new ones, with her 20 per cent Family Death Tax, levied each time a family business passes a generation.
"Every business expects to pay tax, but for Labour to kill off homegrown family businesses is a tragedy. In particular, I have huge empathy for the small businesses and start-ups that will suffer. Labour has shown its true colours with a spiteful budget."
Mr Dyson, 77, employs 3,500 people in the UK.
In Labour's first budget in 14 years, Reeves has tightened up inheritance tax relief on business property. A family company looking to hand assets of more than £1million down to younger generations would now be charged 20%, which is below the standard 40% rate.
Meanwhile, farms worth more than £1million will now be subject to a 20% levy.
The two changes could raise £520million by 2029-2030, the Office for Budget Responsibility calculates.
Previously, farmland and associated buildings had benefitted from 'agricultural property relief'.
However, anyone inheriting working farmland and its buildings will have to pay 20% tax on valuations over £1 million from April 2026 following the Budget.
With farmland alone worth about £10,000 an acre, any estate of over just 100 acres looks set to be liable. - although Reeves says only 27% of farms across the country will be affected.
There are concerns smaller farms could be dragged in when buildings like farmhouses and barns are taken into account.
Kim Gower, who owns Blandred Farm in Folkestone, Kent, is equally aghast and fears the new inheritance tax could force the family to sell off a fifth of their land.
She said: "Agricultural land has been traditionally exempt from inheritance tax to avoid the necessity of selling it on the death of the owner to cover that bill.
"It often takes several generations to build up a farm because land and buildings are so expensive.
"On our family farm, it has taken 100 years to grow it from a small dairy farm to the larger arable farm we have today.
"Even the smallest farms will be unlikely to fall under the £1 million threshold set by the Chancellor.
"It has taken us the last 30 years to increase the farmland by a fifth. We also still have a on this land.
"For us, it effectively means that we will have to sell a fifth of our land to pay the inheritance tax, wiping out three decades of work and investment."
Another devastated farmer whose land has been passed down through generations told of how his family could face an inheritance tax bill of almost £2m.
Martin Twyman, 84, said he was "deeply worried" about the future after Chancellor Rachel Reeves' new budget earlier this week.
Mr Twyman, who owns HW Twyman Farm near Canterbury, Kent, inherited the business from his father Harry who started with a small market garden in the 1930s.
The family have been working on the land and expanding the business since the late 1950s - including with daughter Tracy - to grow wheat, barley, apples and potatoes.
Mr Twyman said: "I would love my family to continue the business that my father started - and for the next generation and their children.
"But the reality is my family could be left with a huge tax bill which would mean having to sell off lots of land - and then it would make it unviable."
"What we are talking about is growing our own food in the UK, which is surely a basic need, but now faces this new threat."
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