Labour has broken its 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 a year.
First Minister Nicola Sturgeon described the move as a "broken promise of the worst kind".
Under Labour's plans, income tax would rise by 1p across the board.
When Labour's announced their proposals at the beginning of February, they 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 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."
Challenged by Ms Sturgeon during STV's live leaders' debate on Tuesday night, Labour's leader in Scotland failed to outline further protection for low earners, having failed for the last two months to explain how the proposed £100 rebate would be delivered.
The SNP leader said:
"This is a broken promise from Labour of the worst kind.
"It is bad enough that Labour want to put up taxes on 2.2 million basic rate taxpayers including 500,000 pensioners. To fail to offer any protection to those on the lowest earnings would be a true betrayal."
"Labour has clearly made promises to taxpayers that they are neither willing nor able to keep."
Kenneth Gibson added:
"Prior to 01 February Labour opposed any tax increases before suddenly u-turning and demanding a rise of 1p across the board. It was then suggested that pensioners and the low paid queue up a year later for a £100 rebate from the council.
"Labour's policy was so obviously ill-thought out that they have already abandoned it and decided just to take more money out of the pay-packets of people on below-average earnings.
"It is clear that Labour are clueless on tax, with an absence of coherent policies and no vision for Scotland."