Boris Johnson is facing an onslaught of criticism from Tory rivals for No 10 after it was revealed his £10bn pledge to cut income tax would hand the biggest sums to rich pensioners – and £3,000 to himself.

As the leadership race turned nasty, the frontrunner was also accused of a Brexit plan that would “annihilate” the party and put Jeremy Corbyn in power “by Christmas”, and of “hiding” from scrutiny.

The backlash came as 10 of the 11 candidates made the cut ahead of the first round of voting on Thursday, after successfully securing the required backing of eight fellow Conservative MPs.

Michael Gove insisted he was still “in it to win it”, but continued to be dogged by the revelation of his past cocaine use, leaving Jeremy Hunt – who unveiled Amber Rudd and Penny Mordaunt as heavyweight new supporters – as Mr Johnson’s likely biggest rival.

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The focus of the contest turned from Brexit to tax after Mr Johnson extraordinary pledge to hike the threshold for the 40p tax rate from £50,000 to £80,000, partly funded by higher national insurance (NI) payments.

Mr Gove tore into the plan, saying: “One thing I will never do as prime minister is to use our tax and benefits system to give the already wealthy another tax cut” – while Dominic Raab, another leadership contender, called it “protecting privilege”.

Whitehall figures seen by The Independent show the biggest winners would be the wealthy elderly – the bedrock of the Tory membership who will pick the next prime minister – because they pay no NI.

The ten contenders in the Conservative leadership race (top row, left to right) Boris Johnson, Dominic Raab, Jeremy Hunt, Rory Stewart and Mark Harper, (bottom row, left to right) Esther McVey, Matt Hancock, Andrea Leadsom, Michael Gove and Sajid Javid (PA)

Those earning £75,000 or more would be handed up to £5,000 a year, twice the reward for non-pensioners earning that amount, while Mr Johnson would enjoy a £3,000 boost on his £300,000-plus annual earnings.

No one earning under £50,000 would enjoy a tax cut, prompting one Whitehall source to say: “At a time when the Conservative Party is losing younger voters, Boris' measure feels tone deaf.”

There was fury in Scotland, where – because tax rates are devolved, but NI rates are still set in London – anyone earning between £50,000 and £80,000 would face a tax hike.

Mr Johnson’s claim that the £10bn bill would be partly met from a pot set aside for a no-deal Brexit was branded “absurd” by the Institute for Fiscal Studies, which pointed out it would come from higher borrowing or spending cuts.

The favourite also found himself in the firing line over his readiness to crash out of the EU in October, as both Mr Gove and Mr Hunt issued identical, apocalyptic warnings about the consequences.

The foreign secretary said MPs had “made clear” they would block no-deal, which would force Mr Johnson to “call a general election to change the parliamentary arithmetic”.

“The lessons of the European and Peterborough elections are clear. If we fight an election before delivering Brexit, we will be annihilated,” Mr Hunt said, at his campaign launch.

A few hours later, Mr Gove echoed the warning, saying: “There would be a vote of confidence in the House of Commons that the government would lose, there would be a general election. We would have Jeremy Corbyn in Downing Street by Christmas,” he said.

The environment secretary also took a further swipe at Mr Johnson by arguing his party must not elect a leader who will “hide in their bunker”.

The former foreign secretary is facing growing criticism for failing to open himself up to scrutiny, having refused to take part in broadcast interviews or, so far, stage a campaign launch.

Both rivals made clear they would seek further Article 50 extensions, rather than be forced into an election by embracing no deal, but declined to set their deadlines for delivering Brexit.

Mr Gove faced suggestions that his cocaine admission had doomed his campaign and, unlike Mr Hunt, was unable to unveil new backers and the appearance of momentum.

But he hit back, saying “I’m in it to win it” and pointing to previous occasions he had been written off, both during the Brexit referendum and in his big cabinet jobs.

“Every time I’ve been given a job I’ve been told it’s impossible and have delivered,” Mr Gove said – adding he was “sure” he would make it to the final two candidates.

Sajid Javid and Matt Hancock completed the list of 10 candidates, ahead of a first round of voting on Thursday.

The list will be whittled down to just two by multiple votes of the 313 Conservative MPs by the end of the month, before the final choice is made by the 160,000 Tory members.

The new prime minister will be known by the end of the week beginning 22 July – leaving open the possibility that MPs will already be on their summer recess.