In a withering verdict on the two Tory leadership contenders, Hammond suggested that their multi-billion pound spending proposals would trash the party’s reputation for economic “responsibility”.
He also hinted that neither man was being “honest” about the consequences of their tax and spend plans.
The Treasury chief ridiculed Johnson and Hunt’s proposals for quitting the EU without a deal, with each claiming they could spend his £25bn warchest built up to deliver Theresa May’s Brexit plans.
Hammond stressed that the money would simply not be available in a no-deal scenario, at a stroke demolishing the claims made by the two leadership rivals.
Johnson has promised to spend billions more on education and tax cuts for those earning more than £55,000 a year, while Hunt pledged £6bn in payments to farmers and fishermen to cope with the chaos of exiting the EU without any agreement.
Hammond and Theresa May earlier this year predicted that austerity would “come to an end” in the coming year.
But in an interview with the BBC, Hammond explicitly warned that his plans to end nine years of Tory cuts could be put into reverse by Johnson and Hunt.
“They would either require borrowing to be increased way beyond the government’s cap on borrowing, or they’d require cuts in spending somewhere else, or they require increases in tax somewhere else,” he said.
“Of course everybody wants to see the era of austerity behind us. But in order to be able to release the headroom we have built up we have to get a deal as we leave the EU so we that we do so smoothly.
“If we don’t get a deal, all of the headroom and more will be needed to support the economy to deal with the consequences of a no deal exit and the hole that will make in our public finances.”
Hammond said that there “isn’t a pot of money sitting in the Treasury” under a no-deal scenario.
“My concern is that this government has built up a reputation for fiscal responsibility and the British people have worked incredibly hard over a decade now to rebuild our public finances and I think it’s very important that we don’t throw that away.
“If we’re not careful all we end up doing is borrowing more, spending more on interest instead of on our schools and our hospitals and our police, and delivering a bigger burden of debt to our children and our grandchildren.
“We have to live within our means and people have to be honest about the consequences of either spending more money or of cutting taxes. That will have implications for borrowing or for spending and taxes elsewhere.”
Earlier, Hammond infuriated Brexiteers when he tweeted his criticism of the two contenders.
He was swiftly slapped down by a spokesperson for May, who said: “The comments are clearly made in the context of the Conservative leadership contest. Unlike the Chancellor, I have not been commenting on the leadership contest.”
In a sign of the growing backlash among Johnson supporters, junior minister Kwasi Kwarteng - a former ministerial aide to Hammond at the Treasury - dismissed his latest remarks as the outburst of someone on the edge of leaving government.
“Philip Hammond has said a lot about Brexit over the years,” he told Sky News. “I would be very surprised if he were going to be the Chancellor to make these decisions.”
The independent Institute for Fiscal Studies also slammed both Johnson and Hunt’s costings, with director Paul Johnson singling out the foreign secretary’s claim that the UK government had spent a trillion pounds on bankers in the 2008 crash.
“It is simply not true that in any real sense we spent £1tr bailing out the banks in the same way that he’s referring to potentially finding £6bn for the farmers and fishermen,” the IFS boss told Radio 4.
Naomi Smith of the Best for Britain pro-EU campaign said: “With both threatening a no-deal Brexit, and it’s clear they are a two-man wrecking crew who won’t listen to sense even when it comes from their own Chancellor.”
In an interview with Sky News, Hunt admitted that his plan to boost defence spending by £15bn would be “delayed” by a no-deal Brexit.