The contest to succeed Theresa May as the UK’s next prime minister has been branded a “horror show” by Scotland’s First Minister.

Nicola Sturgeon spoke out after frontrunner Boris Johnson unveiled a plan to give a massive income tax cut to higher earners south of the border.

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The former foreign secretary wants to increase the threshold at which people in the rest of the UK start paying income tax at 40p from £50,000 to £80,000.

The change would not apply in Scotland, as the setting of income tax is now devolved to Holyrood ministers.

But it would be part funded by increasing employee national insurance payments, which Westminster has sole control over.

With this rise applying across the UK, SNP MSP Angela Constance said: “Scottish taxpayers now face the prospect of paying for a tax cut for the likes of Boris Johnson and his cronies.”

She added: “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.”

Meanwhile, Ms Sturgeon took to Twitter to voice her concerns.

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The SNP leader stated: “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.”

Her comments came as the race to be the next Conservative leader officially got under way, with some of the candidates vying to succeed Mrs May launching their campaigns.

Michael Gove, the Scottish-born Environment Secretary, is battling to stay in the contest, after his admission of cocaine use led to calls to withdraw.

Fellow leadership hopeful Rory Stewart has already had to apologise for smoking opium at a wedding in Iran.

Meanwhile, Jeremy Hunt, the former health secretary who is now Foreign Secretary, said he was still in favour of reducing the legal time limit for abortion from 24 weeks to 12 weeks.

And both Dominic Raab and Esther McVey have come in for criticism after they refused to rule out suspending Parliament in order to get a no-deal Brexit through.

Setting out his tax plans in his Daily Telegraph column, Mr Johnson said: “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.”

But Ms Constance hit out and stated: “Boris Johnson’s priorities are all wrong – and this latest wheeze is an appalling insight into the future of the country if he gets 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 Conservative leader Ruth Davidson has already backed Home Secretary Sajid Javid in the leadership race.

Ms Davidson said that Mr Javid, the son of a Pakistani bus driver, “embodies the Conservative values of aspiration, education, opportunity, hard work and just reward”.