The SNP is ignoring the “big fiscal challenges an independent Scotland would immediately have to confront,” the Institute for Fiscal Studies has said.

In their assessment of the party’s manifesto, the think tank accused John Swinney’s party of disregarding the “potential hit to economic growth from leaving the UK.”

David Phillips, an Associate Director at the body, also questioned some of the calculations used in the 35 page policy prospectus.

The comments could be embarrassing for the party, with the First Minister repeatedly using IFS analysis of Labour spending plans to criticise Sir Keir Starmer.

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The SNP manifesto made a number of calls for the next UK Government to increase spending in a number of areas.

On the NHS, the SNP argues that an extra £16 billion for the health services in England would generate an extra £1.6 billion for the Scottish Government.

However, Mr Phillips points out that “because some of the increases in spending in the rest of the UK would be funded by income tax rises that wouldn’t apply in Scotland, the Scottish Government’s own funding would not increase by as much as the SNP appears to assume.” They also call for at least £28bn to be invested in the green economy, a £20bn “essentials guarantee” in the benefits system, plus annual uplifts in support for housing costs and the abolition of the two-child limit.

The SNP argue that the cost of all this could be met by big UK-wide tax rises for higher earners, additional economic growth from the UK rejoining the EU in the coming parliament, and additional borrowing.

The party argues that if the rest of the UK was to match Scotland’s tax rates, the exchequer could add an estimated £16.5bn to its coffers in 2028-29.

The party also claims that the UK rejoining the EU would see an annual £30bn boost to economic growth.

In his analysis, Mr Phillips writes: “TThe SNP manifesto calls for UK-wide spending plans to be topped up.

“This is to avoid the need to cut spending on unprotected areas, and to increase spending on, in particular, the NHS, working-age benefits, overseas aid and green investment.

“They argue that the cost of this could be met by UK-wide tax rises, additional economic growth from the UK rejoining the EU in the coming parliament, and additional borrowing.

“However, in its call for Scottish independence, the SNP ignores the potential hit to economic growth from leaving the UK, and the big fiscal challenges an independent Scotland would immediately have to confront.”

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He questioned the SNP’s calculations on extra spending for the NHS, saying that as “some of the increases in spending in the rest of the UK would be funded by income tax rises that wouldn’t apply in Scotland, the Scottish Government’s own funding would not increase by as much as the SNP appears to assume.”

Mr Phillips says the SNP tax plans would see income tax for someone in England, Wales and Northern Ireland earning £50,000 a year rise by £1,600, while those earning £125,000 would see an increase of £5,200.

He also states that “in the seemingly unlikely event that the UK did rejoin the EU within the next parliament” the party’s suggestion that his would lead to a £30bn annual boost to growth “would not be an unreasonably high figure for the eventual boost to revenues.”

Mr Philips also says there is some merit in SNP plans for further tax devolution to Scotland.

“Devolving income tax on savings and dividends is a sensible idea, as is devolving National Insurance contributions,” he writes. “However, devolving all income tax rules would add complexity for some taxpayers and the tax authorities.

“And VAT is a particularly tricky tax to devolve, potentially creating trade barriers between Scotland and the rest of the UK, with reforms to VAT in Scotland also complicating the system.”