LABOUR has been accused of breaking a promise to protect low paid workers as part of its plan to increase the basic rate of income tax.

The party confirmed it had dropped plans to hand a £100 rebate to workers earning less than £20,000, insisting changes to the personal allowance - the starting point for paying income tax - had made the move unnecessary.

But the SNP and Conservatives launched furious attacks on Labour, with Nicola Sturgeon describing the move as a "broken promise of the worst kind".

Ruth Davidson, the Scots Tory leader, said hiking basic rate income tax without the rebate meant Labour was denying the lowest paid the boost they received in the Budget, when the personal allowance was increased from £11,000 to £11,500 from April next year.

The row exploded as tax continued to dominate campaigning for the Holyrood election on May 5.

Under Labour's plans, the basic and higher rates of income tax would rise by 1p and the additional rate, paid by those earning over £150,000 by 5p.

When Kezia Dudgale, the Scots Labour leader, unveiled the proposals at the beginning of February, she pledged to give a £100 rebate to all those on less than £20,000.

At the time, Labour explained it was intended as a one-year stop gap measure until Holyrood gained greater control over income tax from April next year.

A party briefing stated that in future years "we will look to use the tax system more directly to ensure low earners continue to be net winners from our tax plans."

But, challenged by Ms Sturgeon during STV's live leaders' debate on Tuesday night, Ms Dugdale failed to outline further protections

for low earners beyond the changes to the personal allowance already announced in the Budget.

The SNP leader said: "This would be a broken promise from Labour of the worst kind.

"It is bad enough that the Labour party wants to put up taxes on 2.2 million basic rate taxpayers including 500,000 pensioners.

"But to fail to offer any protection to those on the lowest earnings would be a true betrayal of Labour’s roots."

She added: "Labour have clearly made promises to taxpayers that they are not willing or able to keep."

Scots Tory leader Ms Davidson said:

"Kezia Dugdale's plan to tax everyone more, then ask the low paid to queue up a year later for a £100 rebate from the council was so ludicrously ill-thought out that even she has now decided to scrap it.

"Unfortunately, in its place she has decided just to take more money out of the pay-packets of those on below-average incomes.

"Just this week, workers across Scotland are due to receive some relief with the increase of the personal allowance – now Kezia Dugdale wants to take that relief away."

The Liberal Democrats, who like Labour back a 1p rise in the basic and higher income tax rates, called on Labour to explain how it would spend the extra revenue of around £500million.

Scottish Labour accused the SNP of trying to divert attention from its own programme of modest tax cuts, which

has been attacked as "timid" and insufficient to protect public services from spending cuts.

A Scottish Labour spokesman said:

"The original payment of £100 was for one year to ensure that those earning under £20,000 would not pay more than today.

"We always said this was for 2016/17 only.

"We promised to protect those on low incomes and to make sure those with the broadest shoulders pay the most and that is what our plans do.

"Under our plans those earning under £20,000 won't pay a penny more than they pay today because of the recent changes being made to the personal allowance.

"Our decision to increase the top rate, and to maintain the threshold for higher rate tax payers as it is today, means the wealthiest will pay the most to stop the cuts."