Adopting a "do nothing" approach to income tax in Scotland and keeping the levy in line with the rest of the UK would be "significantly regressive" and provide a financial boost for top earners, a think tank has said.

Proposals for a 1p rise in the charge, as put forward by Scottish Labour and the Liberal Democrats, would increase tax for most Scots but would "hit richer earners harder than the poorest", it added.

With Holyrood now having limited powers over income tax as a result of the 2012 Scotland Act, IPPR Scotland looked at the impact of the changes proposed by different parties.

Director Russell Gunson said: "Under existing powers, the Scottish Parliament may be facing a 'devil's choice' between doing nothing and thus allowing tax cuts for higher earners, or increasing taxes across all bands, including for middle and potentially lower earners."

Labour plans for a 1p rise with annual rebates of £100 for low income tax payers would see the high earners face the largest increase, with middle earners facing a small rise, while the poorest 30% of households could be better off, IPPR Scotland said.

But it stressed "further clarity" is needed from Labour over how the rebate would work.

Liberal Democrat plans would see all but the poorest 10% having to pay at least a marginal increase in tax, the think tank added, while UK Government proposals to raise the higher rate threshold would cut tax for those earning over £43,000, and "would be regressive with the highest-earning households seeing significant cuts in their income tax by 2020-21".

Middle earners would see "very small tax cuts", it added, while there would be "no benefit" for the poorest households.

The SNP has rejected calls for income tax in Scotland to be increased by 1p in 2016-17. Both it and the Greens will set out their proposals for how they would use further tax-raising powers coming to Holyrood in due course.

Mr Gunson said: "From our analysis we can see that under existing powers, doing nothing over the next four years will lead to tax cuts for the highest earners in Scotland as UK Government tax cut plans are passed on to Scotland.

"We find that this would be significantly regressive with the highest-earning households benefiting from tax cuts worth hundreds of pounds a year.

"Assuming new powers do come to the Scottish Parliament as planned, then the same would be true if those powers are used to match UK Government plans on tax in Scotland."

He added: "Scottish Liberal Democrat and Scottish Labour plans would hit richer earners harder than the poorest, with the highest earners paying the most.

"However, Scottish Liberal Democrat plans would see all but the poorest 10% of households pay at least marginal increases in tax. Scottish Labour propose a payment of £100 to try to get around this problem.

"If this is possible, and implemented, then the rebate would see the poorest 30% of households in Scotland benefit from increases in income, and would reduce the tax increase further up the income spectrum."