Boris Johnson and Jeremy Hunt have been accused of misleading the public with “extraordinary” tax-and-spending pledges, as leading economists and senior Tories unite in criticism.

The two Tory leadership candidates came under fire after Mr Hunt unveiled a no-deal Brexit spending splurge worth almost £20bn – while a Johnson ally promised big public sector pay rises if the favourite wins.

The spending race provoked alarm from Conservatives, including Philip Hammond, the chancellor, and the former leadership contender Rory Stewart, who warned that such promises would make it impossible to attack Jeremy Corbyn for his “unfunded” pledges.

The head of the respected Institute for Fiscal Studies (IFS) went further, saying the two candidates were misleading voters in claiming they could dip into a £27bn “war chest”.

Paul Johnson pointed out it was a figure for one year only, so could not be used for permanent tax-and-spending changes – and it would not be available at all if the UK crashes out of the EU.

“There have been some extraordinary pledges – they add up into the tens of billions of pounds,” the IFS director said.

“They claim, somehow, that these will be paid for from this so-called Brexit war chest. Well, they are not going to be.

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“First, that is only available in the event of no deal not happening. And, in any case, what they are just saying is they are willing to borrow more.”

Mr Hunt, as he set a new deadline of 30 September for a no deal becoming inevitable, pledged £6bn to compensate some industries from tariffs – claiming £1 trillion had been spent to bail out the banks.

But Mr Johnson said: “It is simply not true that, in any real sense, we spent £1 trillion bailing out the banks in the same way that he’s referring to potentially finding £6bn for the farmers and fishermen.”

And, on public-sector pay, he pointed out the freeze was over anyway – arguing the cash now being spent would be in jeopardy from a no-deal Brexit because “the economy will grow less quickly”.

Mr Stewart called for “restraint”, tweeting: “Conservatives cannot criticise Jeremy Corbyn for unfunded tax and spending promises if we then make them ourselves on top of a no-deal Brexit.”

Chris Patten, a former Tory chair, echoed the criticism, saying: “The debate is awful, making John McDonnell, with all their tax and spending pledges, making Labour look like fiscal moderates.

“How we are going, in future, if it comes to an election Labour against the Conservatives, to make a case against Labour as being irresponsible, heaven alone knows.”

The IFS had already totted up Mr Hunt’s campaign pledges – also including a huge leap in defence spending to at least £40bn, even before he announced the £6bn tariffs bailout plan.

Similarly, it has put the bill for Mr Johnson’s eye-catching vow to slash taxes for top-earners, plus help for the lower-paid, at £20bn, with the wealthy gaining by far the most.

Philip Hammond, who is certain to be sacked when Theresa May leaves on 24 July, also pointed out his “war chest” would be needed to mitigate a crash-out Brexit.

“The ‘fiscal firepower’ we have built up in case of a no-deal Brexit will be available for extra spending only if we leave with an orderly transition,” Mr Hammond tweeted.

“If not, it will all be needed to plug the hole a no-deal Brexit will make in the public finances.”

The pair’s spending promises “would either require borrowing to be increased way beyond the government's cap, or they'd require cuts in spending, or they require increases in tax,” Mr Hammond told the BBC later.

Jonathan Portes, the professor of economics at King’s College London, told The Independent Mr Johnson and Mr Hunt were correct to plan higher spending to respond to a “severe economic shock”.

But he said: “I have to agree with Philip Hammond that the right way to decide on taxes and is through a Budget and spending review, not a Tory leadership auction of badly targeted tax cuts.”

Labour made hay with the growing criticism, turning the tables on their opponents by accusing them of “reckless spending commitments”.

“The spending promises being thrown around by both Johnson and Hunt once again prove that austerity was a political choice for the Conservatives and not an economic necessity,” said John McDonnell, the shadow chancellor.