A hard border would be required on the island of Ireland in the event of a 'No Deal' Brexit according to the British government.
Official documents leaked to the Sunday Times show Downing Street expects a return to border checks as current plans to avoid them will prove "unsustainable".
The documents also show Britain will face shortages of fuel, food, and medicine if it leaves the EU without a transition deal.
The newspaper says the forecasts set out the most likely aftershocks of a 'No Deal' Brexit rather than worst-case scenarios.
It comes ahead of Boris Johnson's first EU trip this week during which he will demand a new withdrawal agreement and warn how serious he is about pursuing a no-deal Brexit if he does not get one.
The dossier shows that 85% of lorries using the main channel crossings "may not be ready" for French customs, meaning disruption at ports would potentially last up to three months before the flow of traffic improves, the Times said.
The Times said: "The dossier offers a rare glimpse into the covert planning being carried out by the government to avert a catastrophic collapse in the nation's infrastructure.
"The file, marked 'official-sensitive' requiring security clearance on a 'need to know' basis is remarkable because it gives the most comprehensive assessment of the UK's readiness for a no-deal Brexit."
