Rachel Reeves has been humiliated and left “completely isolated” after a senior tax expert who previously supported her tractor tax published new evidence revealing it will clobber hardworking farmers.

Dan Neidle, a Labour supporter, had spent much of the past week backing Treasury figures regarding how many farms will be affected by the changes to Inheritance Tax.

He was cited by Labour MPs in defence of the policy, and held up by the BBC as proof that farmers are wrong to believe they will be hurt by the tax hikes.

However in a major U-turn this morning, Mr Neidle published detailed new evidence revealing that the Budget will clobber ordinary farmers, while failing to go after the mega-rich tax avoiders it purports to.

After expensive analysis, he has concluded that the few hundred mega rich people who have bought farm estates to take advantage of the current tax relief will be “unaffected by the Budget changes”, while the 125 estates worth more than £1.5 million will only be “somewhat affected” and are “not being hit hard enough”.

He admits that the number of farmers affected by the tax will be much higher than previously thought.

Mr Neidle concludes that the Budget does nothing to stop rich people taking advantage of the current low taxes for farmers, and that the cap should be raised “dramatically” so that only the wealthiest farms become subject to inheritance tax.

The research also casts huge doubts on the Treasury and OBR’s figures regarding how much the tax will actually raise for the country’s coffers, with Mr Neidle concluding they look “optimistic”.

In plain terms, he warns: “Some individual farmers won’t be able to plan, and will pay too much.

[While] people who aren’t farmers will keep using farmland as an IHT planning vehicle, comparatively unaffected by the Budget.”

The research will pile even further pressure on Rachel Reeves to announce a revision of the policy.

On Thursday it was claimed that the Treasury is now working on a climbdown over aspects of the tax hike, however this was vigorously denied by the Treasury and Downing Street.

Responding to Mr Neidle’s findings, the Countryside Alliance said it leaves the Chancellor “completely isolated”.

Chief Executive Tim Bonner said: “The Treasury’s chief cheerleader Dan Neidle executing a rapid reverse ferret on the #FamilyFarmTax. Who would have known that farmers might have understood the economics of farming?

“The Chancellor is now completely isolated on this policy. Every other political party is opposed, but more importantly public opinion is heavily on the side of farmers.

“Now that Mr Neidle and others have accepted that the Treasury’s numbers don’t add up the Chancellor has an opportunity to resolve what will otherwise become an increasingly angry and divisive campaign.”

He said he now feels there is “some light at the end of the tunnel”, and said he remains hopeful that the Family Farm Tax can ve resolved.