Napoleon once said it was better for generals to be lucky than good, and by that score, Theresa May picked well when she chose Philip Hammond as her chancellor.

As the Institute for Fiscal Studies made clear in its post-budget analysis, Hammond just got very lucky indeed. In the run-up to the budget, Britain’s leading tax and spending experts said the chancellor had been put in a real bind by the prime minister and would only be able to fund extra investment in the NHS by raising taxes, borrowing more or taking the axe to spending elsewhere.

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The Office for Budget Responsibility – the independent body responsible for official economic and financial forecasting – solved that problem when it said the public finances were in better shape than it previously thought, with the result that public borrowing would be £12bn lower this year and down in every subsequent year as well.

That meant the chancellor could fund his NHS increase, cut income tax and make universal credit more generous – all while leaving his borrowing in the mid-2020s unchanged. Given that the OBR is forecasting Britain’s pattern of mediocre growth will continue for the next five years, this all looks too good to be true.

Gain or loss as % of net household income Gain or loss as % of net household income

Which, as far as the IFS is concerned, it probably is. The thinktank said there is no guarantee lady luck will continue to smile on the chancellor and he may come to regret a budget described by the IFS director, Paul Johnson, as a “bit of a gamble”.

Chancellors can’t always rely on the OBR finding £12bn down the back of the sofa. Since Hammond arrived in the Treasury in the summer of 2016, the OBR has twice revised its borrowing forecasts upwards – once as a result of the referendum and once because it despaired of the UK’s dismal productivity record improving.

Successive forecasts for borrowing, £bn Successive forecasts for borrowing, £bn

The IFS thinks there is a one in three chance of forecasts for the public finances deteriorating significantly over the next year, which would leave Hammond in an even deeper hole than the one he has just emerged from. The government’s lack of a parliamentary majority would rule out tax increases and it would be politically unthinkable to tell the country that, despite everything, austerity was starting up again.

Johnson’s view that borrowing would take the strain is supported by the fact that Hammond – and other chancellors before him – tend to respond to good and bad breaks in quite different ways. When the OBR said things were looking worse in 2016 and 2017, the chancellor responded by letting borrowing rise. Now the OBR has provided a pleasant surprise, he has spent more and left borrowing unchanged. “Keep doing that,” Johnson said, “and the deficit can only go one way.”

Current receipts, as a percentage of GDP Current receipts, as a percentage of GDP

The IFS also scoffed at the idea that Hammond’s budget represented a spending bonanza. Cuts were not about to reversed and, once adjusted for a rising population, the budgets of those departments not protected by the Treasury’s ringfence would be lower in five years than they are today.

Eight years of austerity has returned public spending as a share of national income to its level 15 years ago, while government tax receipts as a share of GDP were last higher in the mid-1980s. IFS analysis shows Hammond’s boost for universal credit has not really altered the picture of the tax and benefit changes since the 2015 election, which have hit the poorest hardest.