BORIS Johnson has come under fire after proposing a £10 billion tax cut for high earners in Middle England paid for in part by taxpayers in Scotland as Nicola Sturgeon condemned the Conservative leadership race as a "horror show".

The SNP attacked the Johnson tax plan as “entirely indefensible” while Tories themselves suggested the Conservative leadership frontrunner’s plan was “not thought through” and would be “detrimental” to the Union.

The controversial move from the former Foreign Secretary came as the fortunes of his main rivals appeared to be rising and falling.

READ MORE: Nicola Sturgeon brands Conservative leadership contest a ‘horror show’

Jeremy Hunt sought to portray himself as the competent unity candidate, who could deliver on Brexit; failure to do so, he warned, would see the Tories “annihilated” as a general election.

Having received a boost by securing the backing of Remainer Amber Rudd, the Work and Pensions Secretary, he secured another one with the surprise endorsement of Leaver Penny Mordaunt, the Defence Secretary.

Michael Gove, meanwhile, has seen the bookie’s slash his odds of a victory following the revelation that he used cocaine on several social occasions some 20 years ago.

But MPs pointed out that what was more damaging was the Environment Secretary’s “hypocrisy”; having at the time condemned the liberal elite in south Hampstead for being able to cope with the consequences of illegal drug use, when it was less easy for ordinary folk living in South Shields. He is due to hold his own campaign launch this afternoon.

Baroness Warsi, the former Tory Chairman, said it would be "hypocrisy of the highest order" for Mr Gove to remain in the contest after his 1999 article in which he criticised "middle class professionals" who took drugs was republished.

As the Tory leadership contest begain in earnest, Mr Johnson launched his tax pledge in the pages of the Daily Telegraph. It would see income tax bills for people earning more than £50,000 a year cut, if he won the Tory crown.

"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," said the London MP.

His proposal would involve raising the 40p tax threshold for high earners from £50,000 to £80,000.

The former Cabinet minister suggested the £10bn annual cost would be paid for by using some of the money set aside for a no-deal Brexit and from raising National Insurance Contributions.

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This would mean that the growing differential between middle income earners in Scotland and England would grow even faster by several thousand pounds a year because the Scottish Government now has power over tax rates and sets them at different levels. However, because NICs applies across the whole of the UK, then the Johnson tax cut for England would partly be paid for by taxpayers in Scotland.

One Tory MP told the Herald: “Boris simply hasn’t thought this through. No surprise there.”

Another said: “This is a prime example of a policy which has not been passed through the prism of the Union. How would it work in practice without it being detrimental?”

The SNP’s Angela Constance said Mr Johnson’s “latest wheeze” provided an appalling insight into the future of the country if he got his way.

“This proposal has more to do with appealing to Tory MPs than to meeting the very real challenges facing the UK. Yet, bizarrely, Boris Johnson remains frontrunner to be the next Prime Minister.

“Scottish taxpayers now face the prospect of paying for a tax cut for the likes of Boris Johnson and his cronies.

“That would be entirely indefensible and is only likely to see a further rise in support for independence, which would give Scotland full powers over tax,” added the former Communities Secretary and MSP for Almond Valley.

The First Minister took to Twitter to voice her concerns, saying: "What a horror show the Tory leadership election is. Tax cuts for the richest, attacks on abortion rights, hypocrisy on drugs, continued Brexit delusion. True colours well and truly on show."

At his campaign launch, attended by around two dozen supporting MPs, Mr Hunt stressed how he was prepared to leave the EU without a deal but signalled he could extend Brexit beyond October 31 if an agreement was in sight.

"Without a deal, any Prime Minister who promised to leave by a certain date would have to call a general election to change the parliamentary arithmetic. And that is an election we would lose badly. If we fight an election before delivering Brexit we will be annihilated,” he declared.

The Secretary of State explained that the Conservatives would be squeezed by the Brexit Party on the Right and the Liberal Democrats on the Left.

“We will simply allow Labour through the middle. And, if that happened nationally it would be the end of Brexit," he insisted.

In what is likely to be seen as a swipe at Mr Johnson, Mr Hunt told a gathering said: "We need to get real. We are facing a constitutional crisis. Our new Prime Minister will preside over a hung Parliament. A serious moment calls for a serious leader."

READ MORE: Ruth Davidson backs Home Secretary for Tory leadership

Dominic Raab, at his campaign launch, criticised "bluff and bluster", tried to paint himself as "the conviction Brexiteer with a plan" and also took aim at Mr Johnson.

The former Brexit Secretary spelled out his desire to break from the EU by October 31 even without a deal if necessary when he formally launched his campaign to take over as prime minister.

The 45-year-old called for a "generational change in leadership" as he criticised leadership frontrunner Boris Johnson, who has pledged a tax cut for workers earning more than £50,000.

Asked about his rival’s tax plan, Mr Rabb said he would rather cut the taxes of the lowest paid rather than what would be interpreted as "the caricature that you're the party of privilege and you are only in it to help the wealthy".

Mr Raab had earlier pledged to raise the employee's National Insurance threshold to "take the lowest paid out of payroll taxes altogether".

The Surrey MP vowed to return to Brussels to make a "best final offer" to replace the controversial backstop that has proved a sticking point in Parliament.

And he said he would restore discipline in Government and to bring forward a "Brexit budget" to cope with "this period of uncertainty".

"We're up against it and we won't deliver Brexit with bluff and bluster. I'm the conviction Brexiteer with the plan, the discipline and the focus to lead us out by the end of October," added Mr Raab.

At his campaign launch, Matt Hancock pledged to increase the national living wage to more than £10 an hour.

The UK Health Secretary said this would increase the pay of people on the living wage by £3,500 a year.

Currently a 100/1 outsider to take over from Theresa May as Tory leader, Mr Hancock also pledged to reduce taxes on working people "when we can afford it", draw up a long-term plan for education, and proposed an insurance scheme so people did not have to sell their homes to pay for social care.

He said: "When we can afford it I will reduce tax on working people so they can keep more pounds in their pockets.

"For the lowest paid I will increase the national living wage to two-thirds of the median income, over £10 an hour, by the next general election in 2022.

"My plan will increase the pay of those on the national living wage by £3,500 a year," he added.

Nominations for the leadership close at 5pm. The first vote is on Thursday. It is expected by then the current 11 candidates will have been whittled down to single figures. The first live TV debate is due on Channel Four on Sunday. Other rounds will follow next week. The final two candidates, who will put their cases before the party membership, are set to emerge by June 22. The new leader and PM is set to be announced the week beginning July 22.