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 said the forecasts set out the most likely aftershocks of a no-deal Brexit rather than worst-case scenarios.
Elsewhere, the Sunday Business Post reports that Irish consumers could be faced with fuel supply shortages in the event of a no-deal Brexit.
The Irish Petroleum Importers Association said price hikes are also likely as World Trade Organisation tariffs would be placed on some products imported from Britain.
The newspaper reports that the supply of petrol, diesel and kerosene will be disrupted, with rural Ireland likely worst affected.
Meanwhile, the Mail on Sunday reported that British Prime Minister Boris Johnson has accused the UK's former chancellor Philip Hammond of “gravely damaging” the national interest with his bids to frustrate Brexit.
In a letter seen by the paper, the PM said it was “plain as a pikestaff” that the EU “will simply not compromise as long as they believe there is the faintest possibility that Parliament can block Brexit on 31 October”.
The Sunday Telegraph reported a Downing Street source accusing Tory Remainers of “appalling dishonesty” for attempting to use the Brexit delay to “cancel the referendum”.
The paper also said that Brexit Secretary Stephen Barclay has signed the “commencement order” that will trigger the end of the supremacy of EU law in the UK on October 31.
It comes as Boris Johnson will tell French and German leaders that there must be a new Brexit deal when he meets them next week.
The British PM is heading to Berlin on Wednesday and Paris on Thursday ahead of a G7 summit of world leaders. However, the European Union has previously said it wouldn't renegotiate.