Johnson ordered government departments to "turbo-charge" contingency planning after taking power on July 24, promising "all necessary funding" beyond the £4.2 billion (4.6 billion euros, $5.1 billion) already allocated.
But an analysis of the potential impact of a no-deal departure leaked to the Sunday Times made for grim reading.
Here are the main expected consequences:
Ahead of the previous March 29 Brexit deadline, the government set up "Operation Yellowhammer" to prepare for disruption in 12 key areas, including food and water supplies, healthcare and transport.
Under the plan, every department has an operational centre -- some staffed 24 hours a day -- according to the Institute for Government (IfG).
Meanwhile a huge new publicity drive reportedly costing £100 million is underway to help businesses and consumers prepare.
The government's leaked readiness report found British businesses remain largely unprepared for no deal -- despite a Bank of England survey in March finding around 80 percent judged themselves ready.
The Confederation of British Industry, the main employers' association, has said many goods firms are now less prepared for a disorderly divorce in October, warning that stockpiling will be harder in the busy run-up to Christmas.
In services, some stop-gap agreements have been reached on both sides including a one-year agreement to protect the massive financial derivatives market in London.
The EU says it would immediately begin customs checks, food safety inspections and verification of EU standards at its border with Britain.
Fearing long delays at busy crossing points, Britain has moved to open up new routes and increase links from other ports to ease the pressure on the most important, Dover.
But the Yellowhammer analysis predicts up to 85 percent of lorries using the main Channel crossings "may not be ready" for French customs and could face days of delays.
It states logjams could eventually "affect fuel distribution" nationwide.
Many drugs firms have already built up stocks and changed supply routes to reduce disruption, according to the CBI.
But around 28 percent of food consumed in Britain comes from the EU, and major supermarkets have warned of the risk of disrupted supplies.
The Yellowhammer dossier warns clean drinking water supplies could be impacted, affecting "hundreds of thousands of people".
The EU has agreed to allow flights from the UK into the other 27 member states but only until March 2020.
The high-speed train link through the 50-kilometre (30-mile) Channel Tunnel running between Britain and France will be kept open for three months, as long as Britain adopts EU rail safety standards.
But Whitehall's leaked assessment warns of likely passenger delays at the Eurostar station at St Pancras and Eurotunnel terminal on England's south coast, as well as at airports.
Britain says it will scrap tariffs on most imports, retaining a reduced rate for some agricultural products to protect farmers.
But the EU will treat Britain as a "third country" imposing tariffs that are on average low -- around 1.5 percent -- but bigger in certain sectors: for cars, the rate is 10 percent.
Britain will also lose access to major markets covered by EU trade agreements, although it has replicated some of these, including with South Korea and Switzerland.
A key element of the draft Brexit deal with the EU was protecting the rights of 3.5 million European citizens living in Britain, and over one million Brits in the bloc.
Many EU countries have moved to protect Britons' rights anyway, while more than 800,000 Europeans have already received some form of new "settled status" in Britain.
Without a deal, British citizens travelling to the EU will be limited to 90-day stays and subject to tighter passport checks.
Britain would not immediately apply customs checks on the border with EU member Ireland, to avoid raising tensions in once violence-plagued Northern Ireland.
But the government's leaked analysis concedes that is "unsustainable" and a hard border would return, while the EU has said it will monitor goods moving across what will become its external frontier.