Ruth Davidson told her MPs must block Boris Johnson's tax plan

Two of the group - Colin Clark and Ross Thomson - are backing the former Foreign Secretary in the race to be the next Tory leader and Prime Minister.

Mr Johnson caused an outcry on Monday when he outlined plans to give a tax break to people in the rest of the UK by raising the higher rate threshold from £50,000 to £80,000.

The point at which employee national insurance contributions (NICs) drop from 12 to 2 per cent would also increase to £80,000, adding up to £3000 to people’s NIC bills.

South of the border, this would be offset by a £6000 drop in income tax, a net cut of £3000.

But in Scotland, because income tax is devolved, high earners could see their NIC bills go up without any accompanying cut in tax, leading to a net rise in bills of up to £3000.

READ MORE: Boris Johnson's tax pledge could widen UK-Scottish pay gap by £8k

The SNP said Mr Johnson wanted to use Scots to fund tax cuts for the rich in England.

The respected Institute for Fiscal Studies yesterday said that was simply "wrong", as under the Fiscal Framework, the complex set of rules governing devolved taxes, Scotland's block grant from Westminster would in fact rise under Mr Johnson's plan.

It said SNP ministers could use the extra for income tax cuts or for public spending.

Associate IFS director David Phillips said: “The Scottish Government actually gets a budget boost because of the income tax cut in the rest of the UK. It is, initially, getting the same revenues from its own income tax, and more in block grant funding.

“While the immediate effect of Mr Johnson’s plans would be to increase the NICs bills of high earners in Scotland, they wouldn’t represent an unfair grab of Scottish money.”

Despite Mr Johnson’s plan potentially increasing Holyrood’s budget, Mr Mackay said Ms Davidson must act to stop it, and live up to previous Tory boasts about her MPs acting as a block to protect Scottish interests.

He said: “Boris Johnson’s plans are a disaster for Ruth Davidson - it is a policy that proves once and for all that Scotland simply doesn’t feature in Westminster Tory thinking.

“Ms Davidson has every reason to be worried about the prospect of Boris Johnson becoming Prime Minister - but she seems far more interested in what it means for her own political career than how it will affect the people of Scotland.

“The SNP has used the tax powers at our disposal to ensure that Scotland is the fairest tax part of the UK, and for a majority of people it is also the lowest tax part of the UK.

“Boris Johnson’s plans mean that, not for the first time, our efforts to make Scotland fairer and more prosperous will be undermined by a reckless, right-wing Tory government at Westminster."

READ MORE: Boris Johnson branded 'bigger threat to Union than SNP' after he reveals tax cut plan

Mr Mackay went on: “After the last Westminster election the Scottish Tories made much of the idea that they would vote as a block in the House of Commons to protect Scotland’s interests - but there has been no evidence of this so far.

“They were silent when their bosses handed over £1bn to Northern Ireland to buy off the DUP - but they simply cannot remain silent in the face of this Boris Tax.

“Ruth Davidson must immediately make clear – does she support Boris Johnson’s tax hike? If she doesn’t, will she instruct all 13 or her MPs to vote against any UK budget in which these outrageous tax plans are included? Or will she simply fall into line with the Tory leadership and forget about her previous statements, as she so often does?”

A Scottish Conservative spokesman said: “The SNP should worry about its own tax agenda, which punishes thousands of Scots, before looking at anyone else.”

Mr Johnson's tax plan is a useful distraction for Mr Mackay, who today faces a grilling by Holyrood’s finance committee over a looming £1bn in Scotland’s budget due to forecasting errors.

MSPs want to know how he plan to cope with “negative reconciliations” of £229m in 2020/21, £608m in 2021/22 and £188m in 2022/23.

The Scottish Fiscal Commission has warned the Scottish Government’s borrowing powers will not cover the adjustments, making tax rises or spending cuts likely.

The reconciliations are the result of forecasting errors by the Fiscal Commission and the UK Office of Budget Responsibility when they used estimates to determine previous Scottish budgets.

Although not the Scottish Government's fault, they are now its responsibility to address.

READ MORE: Tom Gordon: Ministers need to look their budget fears in the eye

Labour MSP James Kelly said: “Derek Mackay must produce some answers today over how he intends to close the £1bn black hole in the SNP government’s books. This is a huge financial danger for the devolved government which puts lifeline public services at risk.

“It is time for Derek Mackay to stop the constitutional rants and get real and deal with the looming financial problems he has helped create.”