Johnson, the favourite to replace Theresa May as Tory leader, pledged that if he became prime minister he would raise the threshold for the 40p tax band from £50,000 to £80,000 for workers in England, Wales and Northern Ireland.
As income tax is devolved but National Insurance is reserved, critics of the plan have pointed out it could leave taxpayers in Scotland funding a handout for well-off taxpayers in the rest of the UK.
Labour MP Ian Murray today challenged Scottish Tory leader Ruth Davidson to distance herself from the proposal.
“When so many people across the UK are suffering in poverty, it is sickening that his priority is massive tax cuts for the wealthiest in society.
“The fact that hard-pressed workers in Scotland would pay towards the tax cuts is a double blow, and will create deep divisions within the Union.
“The Tories are playing fast and loose with the Union and can’t be trusted to protect Scotland’s place in the UK. Ruth Davidson must urgently distance herself from this insulting proposal and explain to Scots what she is planning to do about it when he becomes Prime Minister.”
The higher rate of income tax currently applies on earnings over £50,000 in England and the move could benefit more than three million people.
Johnson claimed the cost of the policy could be met through some of the cash set aside for no-deal Brexit planning.
In his regular Daily Telegraph column, he said: "We should be cutting corporation tax and other business taxes.
"We should be raising thresholds of income tax - so that we help the huge numbers that have been captured in the higher rate by fiscal drag."
The move will cost an estimated £9.6 billion a year and will be funded from the £26.6 billion of "fiscal headroom" currently set aside by the Treasury for no-deal preparations.
It will also be partly offset by increased National Insurance contributions.