A leading think tank has accused Nigel Farage’s Reform of poisoning the debate around painful economic choices, as it warned it would be a “considerable surprise” if there are no further tax rises in the next five years.

Paul Johnson, the director of the Institute for Fiscal Studies, slammed the Tory and Labour manifestos, saying they leave voters “guessing”.

But he also hit out at Reform and the Greens for “unattainable” tax claims he said had made offerings from other parties look “feeble”.

Their pledges were helping to “poison the entire political debate”, he said.

The attack comes as Mr Farage faces a growing backlash on another front, over his claim the West provoked Russia’s invasion of Ukraine.

Mr Johnson also accused Labour and the Conservatives of making “essentially unfunded commitments” to improve the NHS as he said a conspiracy of silence between the main parties over difficult decisions after the election was leaving voters in the dark.

Paul Johnson, director of the IFS appearing on the BBC One current affairs programme, Sunday with Laura Kuenssberg. (PA)

There are “painful” economic decisions ahead, “none of which are faced up to” in the manifestos, he said.

These leave voters “guessing” on tax and spending, and on the future size and shape of the state, he added.

The IFS warns several public services are at risk of suffering "sharp cuts" under either a future Labour or Conservative government.

Broadly, the IFS said, the priorities of each party "do not tell us anything about overall spending on each public service".

On the NHS, Mr Johnson said both main parties want to cut waiting times, implement the NHS England workforce plan, build more hospitals and expand mental health services.

Nigel Farage’s party has been accusing of helping ‘to poison the entire political debate’ (ITV Tonight)

But he added: “These “fully costed” manifestos appear to imply all this can be delivered for free. It can’t. You can’t pledge to end all waits of more than 18 weeks, allocate no money to that pledge, and then claim to have a fully costed manifesto.”

He went on: “How would either party deal with backlogs in the court system, overflowing prisons, crises in funding of higher and further education, social care, local government? We have not a clue.”

The IFS says Reform proposes £90 billion of specific tax cuts and £50 billion of spending increases, "paid for" by a £150 billion package of measures that includes substantial, unspecified cuts in welfare and government waste.

Mr Johnson said: “If they want a smaller state – a perfectly reasonable ambition – they should tell us how they will achieve it. We saw the consequences of massive tax cuts with no detail on how they would be paid for in September 2022.”

He said the claim the party could eliminate NHS waiting lists at a cost of £17 billion a year is “demonstrably wrong”.

And he said the large tax cuts would cost “even more than stated, by a margin of tens of billions of pounds per year”.

The Greens want an additional £80 billion a year of borrowing.

“A massive increase in borrowing when the economy is capacity-constrained – and the debt interest burden is already just that, a huge burden – would have unpleasant consequences,” Mr Johnson said.

Their biggest proposal, a £90 billion a year carbon tax, would incentivise a faster transition to net zero but would have “far-reaching economic implications” including making the cost of many essentials more expensive.

“Much, probably most, of any money raised, would need to be used to mitigate those effects, and to support those on lower incomes, not to fund other things. In any case, any effective carbon tax would reduce the amount of carbon-based activity and hence, eventually, raise a lot less,” Mr Johnson said.

Mr Farage has hit back at former prime minister Boris Johnson for labelling his views “morally repugnant”.

His comments came as he unveiled a large poster of a 2016 newspaper front page with the headline “Boris blames EU for war in Ukraine“ at an event in Kent.

Mr Farage said: “Well, perhaps it’s Boris Johnson that’s morally repugnant and not me, I don’t know. But can you see the sheer level of hypocrisy? Can you see the nonsense of all of this?”

Earlier again hit out at the Reform leader’s comments. He said: “You all heard what Nigel Farage said about Ukraine. That plays into Putin’s hands. That kind of appeasement is dangerous for Britain’s security, the security of our allies that rely on us and will only embolden Putin.”

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