Nicola Sturgeon would have to order new income tax grabs on the better off to raise enough cash to combat austerity and meet her own “fairness” test, new analysis has revealed.

Both the First Minister and Finance Secretary John Swinney have refused to use existing powers at Holyrood to boost income tax across the board by 1p, a policy that advocates claim would raise around £500 million and offset cuts to public services.

Despite a string of experts confirming the policy would be “progressive”, Ms Sturgeon has branded it as a “con trick” that would “shift the burden of Tory austerity on to the shoulders of the low paid”.

It has been argued the 1p income tax policy, proposed by Scottish Labour, is fair because the additional amount of tax paid as a proportion of income would be higher for the better off.

But the SNP leader said the income tax bill of a low paid worker would rise by five per cent, or translated to cash terms £7.50 a month for someone on a £20,000 salary, while hers would increase by just 2.7 per cent, or £124 a month in cash terms on a full First Ministerial annual pay cheque of £148,595.

Experts said to ensure tax bills in percentage terms rose equally across the board – if bands remain the same and the basic rate increases by 1p – the SNP would have to advocate increases of 2p on the higher rate, rising towards 3p on the additional rate (for salaries over £150,000) to raise significant sums and meet the party’s own definition of fairness, giving a “1, 2, 3” policy.

Although the Scottish Government would not have power to implement the policy currently, it would be able to bring it in following devolution of full control of rates and bands, which could be in place through the Smith Commission next year.

Under the powers, it would also be able to keep tax static for the lower paid and instead hit only the rich.

However, experts such as Torsten Bell of the Resolution Foundation have argued it would be difficult to raise significant sums under such a move due to relatively low numbers of higher and top rate salaries in Scotland.

David Eiser, a Stirling University economics research fellow and member of the Centre on Constitutional Change, said the SNP’s definition of fairness was “unusual”.

Under this approach, he said, the Upper Rate would have to increase from 40p to 42p and the Additional Rate, currently 45p, would need to increase to 47.25p should the basic rate rise by 1p.

In cash terms, it would mean a taxpayer on £30,000 would pay around £16 a month more, the same as under the plan backed by Labour and the LibDems, while Ms Sturgeon and others on salaries of just under £150,000 would see their tax bills soar by almost £200 a month.

Mr Eiser confirmed that this would be considered a more progressive policy than adding 1p to the Scottish Rate of Income Tax (SRIT), which the Government rejected on the grounds of fairness. However, when asked whether the Scottish Government considered this proposal fair, a spokeswoman said: “This is not what the First Minister said nor is it the policy of the Scottish Government.”

She did not explain the Government’s definition of “fairness”.

Despite attacking the opposition for the “unfairness” in their tax approach due to a disparity over percentage increases in tax bills, Ms Sturgeon’s spokesman refused to confirm that the same rule would apply to the SNP’s own tax policies, which will be unveiled in the coming weeks.

Mr Eiser added: “Fairness is a more subjective concept than progressivity. The SNP argue that adding 1p to the SRIT would not be fair, as those on low incomes would face a proportionately larger increase in their tax bill than those on higher incomes. This additional criterion for fairness is unusual.

“Achieving fairness by this criterion would require each tax band to increase by the same proportionate amount.

“So if the basic rate increased to 21p (5 per cent), the Upper Rate would have to increase to 42p and the Additional Rate would need to increase to 47.25p.

“This would undoubtedly be a more progressive policy than simply adding 1p to all bands. And, as a result, some people might think that it was fairer than simply adding 1p to SRIT. But to argue that a 1p increase in SRIT – a progressive tax policy – is unfair seems an unusual argument for a self-declared ‘progressive’ party to make.”

The Government spokeswoman added: “The Scottish Government will set out the longer term intentions for the further income tax powers, whether or not a fiscal framework has been agreed with the UK Government, before the dissolution of Parliament.”