The IFS predicts that the government will renege on its own rules on annual borrowing against GDP, even before the costs of a no-deal Brexit are accounted for, with its 2020 spending pledges nearly as high as those set out in Labour's 2017 election manifesto.
The think tank said to the BBC "the government is now adrift without any effective fiscal anchor" and that it would be forced to borrow up to $100bn (€112bn).
French president Emmanuel Macron told United Kingdom prime minister Boris Johnson that the remaining 27 European Union members will make their decision by the end of the week after they discuss whether a proposed Brexit deal can "respect" the bloc's principles and is likely to be passed through parliament.
However, in the event of even a "relatively benign" no-deal Brexit, the IFS said that could rise to nearly £100 billion - while debt would climb to nearly 90% of national income for the first time since the mid 1960s.
As the government would look to encourage growth after a no-deal Brexit, the IFS predicts a quick-fire spending spree which would help push up debt to 90 per cent of national income, a ten per cent increase on current levels.
The funding boost announced by Chancellor Sajid Javid last month, changes to the.
As a result, accumulated national debt would climb to nearly 90 per cent of GDP (Gross Domestic Product) for the first time since the mid-1960s, the IFS said.
It found that a no-deal Brexit will hit the United Kingdom economy the hardest and drag down growth. Not only is every spending department about to see a budget increase, we have a Conservative government set to increase day-to-day spending on public services to a level far closer to what Labour promised in its 2017 manifesto than to what was implied by the Conservative manifesto.
The IFS said that a no-deal Brexit could temporarily halt all economic growth for two years, even with "substantial" government spending.
"It will be crucial that these programmes are temporary: an economy that turns out smaller than expected can, in the long run, support less public spending than expected, not more".
This idea is supported by Christian Schulz, the leading United Kingdom economist at Citi, one of the leading firms which helped produce the IFS report.
Of the other scenarios analysed, the IFS found revoking Article 50 would be the best option for the economy, while further delays would suppress economic growth to below 1% in 2020 and less than 1.5% in 2021 and 2022.
Whether - and if so how and when - the United Kingdom leaves the European Union will be perhaps the key determinant of growth over the next few years.
"From a growth perspective, a Brexit deal is a little better, leaving growth at 1.5%, but it would leave no chance of Brexit being cancelled", he said.
