A no-deal Brexit could lead to "consumer panic", rising crime, food shortages and economic chaos, according to an official government document.
The leaked presentation detailed civil servants' assessment of what could happen in the first day, fortnight and month month after Brexit if the UK leaves the European Union without a withdrawal agreement.
The revelations come as departments ramp up their contingency planning for a potential no-deal Brexit, following prime minister Boris Johnson's insistence that Brexit must happen "do or die" on 31 October.
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According to the internal document, which was uncovered by Sky News, the day after Brexit could see UK nationals in the EU lose access to residence rights, slower trade at the border and volatility in the currency and financial markets.
Within the first fortnight, the slide predicted "potential consumer panic and food shortages", and an increased risk of serious organised crime, including people smuggling.
And within the first month, civil servants forecasted "potential law and order challenges" in Northern Ireland, UK residents in the EU having to return to Britain, "unsustainable" pressure on police and a potential 25% drop in the value of the pound.
Downing Street sources said the forecasts, which were drawn up before Johnson entered No.10, were "worst case" scenarios and had not been signed off by ministers.
The revelations came the same day as a second leak to the Guardian showed a Cabinet Office assessment had concluded Whitehall departments were less prepared for a no-deal Brexit than they were in the run-up to the original 29 March Brexit deadline.
An internal document, seen by the newspaper, detailed a number of “reasonable worst-case scenarios”, including an impact on the supply of medicines because of "reduced flow rates across the Dover Straits" and price rises caused by disruption to the food supply chain.
It also said there was a risk tha panic buying would “cause or exacerbate food supply disruption”, parcticularly in light of the added pressure as people prepare for Christmas.
And stockpiles of veterinary medicines would “not be able to match the 4-12 weeks worth... which took place in March, 2019”, it said. It was not financially viable to reduce these risks, and a reduced supply of medicines “would impact the UK’s ability to prevent and control disease outbreaks”, it said.
The document also warned that “low-income groups may be disproportionately affected by price rises" in food, fuel, utilities and services, while food banks may struggle to get enough funding to support those affected.
The Whitehall leaks appear to confirm analysis by the Institute for Government that Whitehall churn means government may never be as ready again for no deal.
As well as saying the UK will leave the EU "come what may" on 31 October, Johnson has ruled out entering into fresh negotiations with Brussels unless the EU agrees to reopen his predecessor Theresa May's withdrawal agreement and scrap the Northern Ireland backstop.
Yesterday chancellor Sajid Javid announced he was making an extra £2.1bn available to help departments' no-deal contingency planning.
And this week, the Department for Transport announced it was setting up a £300m framework to provide extra freight to departments to transport medicines and other critical goods between the EU and UK and Britain and Northern Ireland over a four-year period after a no-deal Brexit.