In his first phone call to Ireland's premier since he took office a week ago, Johnson repeated that the current "backstop" plan to keep the frontier open, which is included in the EU's draft divorce deal, was unacceptable.
"He said that in all scenarios, the government will be steadfast in its commitment to the Belfast Agreement and will never put physical checks or physical infrastructure on the border," a spokeswoman for Downing Street said.
The Belfast or Good Friday Agreement brought peace to Northern Ireland after years of violence which left 3,500 people dead.
Removing border checks was considered a key factor in reducing tensions. But after Brexit, the border will become part of the EU's external frontier, which entails appropriate policing.
'Turbo-charge' no-deal plans
Johnson has promised “all necessary funding” needed to ensure a smooth transition after exiting the EU, even if it requires money beyond the 4.6 billion euros already earmarked. He has called on the government to “turbo-charge” its efforts.
Before the first Brexit deadline on 29 March, the planning for the outcome, called “Operation Yellowhammer”, was preparing for short-term shocks in 12 areas, including food and water supplies, transport, borders and healthcare.
Under the plan, each government department ran an operational centre – some open 24 hours a day, according to the Institute for Government (IfG), an independent think tank in the UK.
The plans were scaled back after the divorce was delayed until 31 October, but now the centres are now starting up again.
Plans include a publicity campaign reportedly costing some 110 million euros to help businesses and consumers prepare for the no-deal exit. Businesses, according to IfG, will need to transition towards “emergency footing”.
Eighty percent of British companies said in a March survey that they were ready for a hard Brexit, but by the end of the month, the Confederation of British Industry (CBI) said that goods firms will have a harder time come October, especially because of the run up to the busy Christmas season.
For companies providing services, stop-gap agreements have been reached between Britain and the EU to protect London’s large financial derivatives market.
Some 3,000 lawyers have also registered in Ireland to ensure that they will be able to practice law across the EU, said the CBI.
Addressing transport woes
The EU has announced it will begin customs and food and safety inspections in order to ensure products are up to EU standards at its border with Britain.
In order to curtail long delays, Britain has opened new routes and increased links from other ports so that Dover will not be overstrained.
Flights routes and schedules from the UK into the 27 member states have been agreed, but only until March 2020. Trucks will also be allowed to keep carrying goods in and out of Britain, also on a time-limited basis.
The high-speed train link running through the 50-kilometre Channel Tunnel that connects Britain and France will be kept open for three months, as long as Britain conforms to EU rail safety standards.
No ferries? No worries
Britain is trying to ensure additional freight capacity for medical devices, medicines and chemicals, although many drug companies have already bolstered stocks and changed supply routes, said CBI.
Earlier this year, these preparations came under the spotlight when it was revealed that the company that had been hired to provide sea transport in fact had no ferries.
Some 28 percent of food consumed in the UK comes from the EU, and supermarkets have warned of shortages while alternate supply lines within the country are found.
The British government plans to retain reduced tariffs for some agricultural products to protect farmers, but will cut tariffs on most imports.
The EU will view Britain as having "third country" status, imposing relatively low tariffs, but higher for some industries, including cars, which will be at 10 percent.
Britain will lose access to major markets under EU trade agreements, but has managed to make similar bilateral agreements with countries such as South Korea and Switzerland.
The EU wanted to ensure that the rights of its 3.5 million citizens living in Britain were protected, as well as the one million Brits in the EU – a key part of the Brexit deal agreed my former prime minister Theresa May in November.
Without a deal, British citizens risk only being able to stay in the EU 90 days and facing tighter passport controls.
Many EU countries have protected the rights of British citizens in their countries, while more than 800,000 Europeans have received some form of settled status in the UK, but there remain many wrinkles to iron out.