Britain will face shortages of fuel, food and medicine if it leaves the European Union without a transition deal, jamming ports and requiring a hard border on the island of Ireland, according to official government documents leaked to the Sunday Times.

The Times said the forecasts compiled by the UK Cabinet Office set out the most likely aftershocks of a no-deal Brexit rather than the worst-case scenarios.

British Prime Minister Boris Johnson's office said it did not comment on leaked documents.

The newspaper said up to 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 UK government also believes a hard border in Ireland will be likely as current plans to avoid widespread checks will prove unsustainable, the Times said.

"Compiled this month by the Cabinet Office under the codename Operation Yellowhammer, 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 Times reported.

"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."

Michael Gove, the minister in charge of coordinating preparations for leaving the EU without a deal, has said the documents are based on "a worst-case scenario".

A scientist from the Economic, Social and Reseach Institute has said the documents indicate the UK "faces a serious risk of a constitutional crisis and of violence spilling onto the streets".

Professor Pete Lunn, a Behavioural Scientist with the ESRI, told RTÉ's Marian Finucane programme that the documents show the scale of the problems facing the UK is remarkable and "that right at the heart of the British government they do know how serious it is".

Mr Lunn said potential consequences for the island of Ireland include shortages, delays and a possible increase in crime.

Mr Johnson has repeatedly vowed to leave the EU on 31 October without a deal unless the EU agrees to renegotiate the Withdrawal Agreement.

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Politicians, north and south of the border, have said the leaked report setting out British preparations comes as no surprise.

After more than three years of Brexit dominating EU affairs, the bloc has repeatedly refused to reopen the agreement, which includes the backstop, a mechanism to avoid a hard border that Mr Johnson's predecessor, Theresa May, agreed in November.

Meanwhile, Brexit minister Stephen Barclay said on Twitter he had signed a piece of legislation which set in stone the repeal of the 1972 European Communities act - the laws which made Britain a member of the organisation now known as the EU.

Though his move was largely procedural, in line with previously approved laws, Mr Barclay said in a statement: "This is a clear signal to the people of this country that there is no turning back [from Brexit]."

A group of more than 100 politicians has written to Mr Johnson calling for an emergency recall of parliament to discuss the situation.

Boris Johnson

"We face a national emergency, and parliament must now be recalled in August and sit permanently until 31 October so that the voices of the people can be heard, and that there can be proper scrutiny of your government," the letter said.

This week, Mr Johnson will tell French President Emmanuel Macron and German Chancellor Angela Merkel that the Westminster parliament cannot stop Brexit and a new deal must be agreed if Britain is to avoid leaving the EU without one.

The prime minister is coming under pressure from politicians across the political spectrum to prevent a disorderly departure, with opposition leader Jeremy Corbyn vowing this week to bring down Mr Johnson's government in early September to delay Brexit.

It is, however, unclear if MPs have the unity or power to use the British parliament to prevent a no-deal departure.

Brexit supporters say there may be short-term disruption from a no-deal exit but that the economy will thrive if cut free from what they cast as a doomed experiment in integration that has led to Europe falling behind China and the United States.