Boris Johnson has promised to take Britain out of the EU with or without a deal on October 31.

However, a secret official memo from the head of the civil service, extracts of which were revealed earlier this year, reportedly warned Johnson of the severe consequences of a no-deal Brexit.

The former chief of the civil service Lord Kerslake, has told Business Insider that the advice should now be published in full, as Britain heads towards a no-deal exit from the EU.

"It's for me uncomfortable that senior politicians in the country are privy to the consequences of something so grave that we as the public haven't seen," Kerslake told Business Insider.

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The former head of the civil service has urged Boris Johnson to release a confidential memo warning ministers about the severe consequences of a no-deal Brexit, which he says includes warnings about "rioting in the streets" if the UK leaves with no deal.

Sir Mark Sedwill, Britain's highest-ranking civil servant, sent a letter to ministers in March this year describing how the country would be affected by a no-deal Brexit.

Extracts leaked in April — just three months before Boris Johnson replaced Theresa May as prime minister — revealed Sedwill's warnings of civil unrest, food prices rising by 10% and a financial crash if Britain leaves the EU without a deal.

His predecessor, Lord Kerslake, who led the civil service between 2012 and 2014, told Business Insider that the 14-page letter, which has never been made publicly available, should be published by Downing Street in full.

"I think in the very extreme circumstances we are in, that advice ought to be published and the prime minister ought to give agreement for it to be published," Kerslake said.

"If he is correct that we can manage the impact of no deal, then he oughtn't to have any difficulty releasing the advice.

He said that it was "uncomfortable" that "senior politicians in the country are privy to the consequences of something so grave that we as the public haven't seen."

The UK is currently scheduled to leave the EU on October 31, and Boris Johnson has pledged to deliver Brexit with or without a deal, despite warnings from business leaders that trade disruption would cause severe problems for many firms.

The leaked extracts of Sedwill's letter outlined various doomsday scenarios, including mass bailouts for businesses, struggling firms unable to secure credit, a harmful currency depreciation, and weakened national security.

Kerslake said the government no longer had adequate time to prepare for leaving without a deal and said it would result in "very serious disruption" which would be "disastrous" for the country and its economic prospects.

The former civil service chief also confirmed he had been told that preparations for other worst-case Brexit scenarios including "rioting in the streets" — which he said in February the civil service was preparing for — were continuing to take place as the UK nears a cliff-edge Brexit.

"There certainly have been conversations about the consequences of rioting," he said, citing conversations with current civil servants who are working on emergency planning measures,

He added that "almost weekly" conversations between central government and local government branches about no-deal planning included discussions about such law and order issues.

Boris Johnson's right-hand man is 'arrogant, absolutist, adversarial'

Dominic Cummings, special advisor for Britain's Prime Minister Boris Johnson, leaves his home in London, Britain August 9, 2019. REUTERS/Peter Nicholls Reuters / Peter Nicholls

Kerslake — who said that a no-deal Brexit now appears to be the "most likely" outcome of Boris Johnson's premiership — also issued a warning about Dominic Cummings, Boris Johnson's most senior adviser.

Cummings — who ran the Vote Leave campaign and has described Downing Street's civil service machinery as "decrepit" — is tasked with delivering Brexit by October 31 but many civil servants are reportedly aghast at his calls for a revolution in Whitehall.

Kerslake said that Cummings is a "very arrogant, absolutist, adversarial person" whose personal qualities would have a "negative" impact on the functioning of the civil service.

"All of those things will have an impact and I think the civil service will try and work with him and manage the impact of all of that. But I don't think they can entirely avoid it having an effect," he said.

Mike Clancy, the general secretary of the Prospect Union, which represents specialist civil servants, said it could be "catastrophic" if Cummings tried to implement radical civil service reforms too quickly before Brexit.

""Dominic Cummings has in the past called for a widespread restructuring of the way the civil service operates. We have already seen, barely half a day into the regime, a step-change in the way accountability operates with special advisers," he told the Guardian.

"At a time when the government is so utterly dependent on its civil service, it would be catastrophic to embark on a distracting reorganisation to address misplaced views about the civil service."