Frictionless trade with the European Union is to end this year, the government has said.
Border checks are “inevitable” after the Brexit transition period ends on 31 December, cabinet minister Michael Gove said.
Businesses say they face extra costs from checks, and an industry body warned the changes could hit food imports.
But officials said firms have enough time to prepare for the changes.
Mr Gove told a Border Delivery Group event: “The UK will be outside the single market and outside the customs union, so we will have to be ready for the customs procedures and regulatory checks that will inevitably follow.”
There will be import checks at the UK border, and traders in the EU and UK will have extra paperwork, the government said.
From next January, all traders will have to fill out customs declarations and be liable to customs checks on goods for cross-channel trade.
If no trade deal is reached with the EU, taxes such as tariffs will also need to be charged and collected.
Facilities such as the Channel Tunnel have been designed for minimal border checks.
New customs infrastructure, facilities and systems as well as staff, agents and vets will have to be in place by the end of this year.
But Mr Gove told the conference there would be light touch administration of trade across the Irish Sea.
However, last week it emerged that Stena Line, the biggest operator of ferries in the Irish Sea, is preparing for trade checks between Great Britain and Northern Ireland.
It was quietly confirmed in a speech. Some might argue it has been inevitable since the election.
But the change in the way the UK trade border functions with our biggest trade partner is one of the single biggest changes to the way the UK economy functions.
Put simply, many industries rely on the frictionless free flow of goods between the UK and the continent.
The unequivocal message from Michael Gove is that businesses should prepare for the the end of that as 2020 draws to a close.
Whereas the impact of all this in the Irish Sea has garnered considerable attention, the new trading arrangements between Dover and Calais and along the Channel Tunnel will have a bigger effect on the economy.
By getting businesses to take the prospect seriously, the government’s hope is that more will be prepared and so delays and disruption can be limited.
But we are dealing with parts of the border that are designed to run without checks.
There will need to be more customs officers, thousands more customs agents, mass recruitment of vets, and new customs posts.
Almost every independent economic analysis – and the government’s own until now – has shown that extra trade friction with what is currently our biggest market will be an overall hit to the economy.
Preparation can help alleviate some of that hit, but not all.
The British Retail Consortium (BRC) said government would have to “move fast”, and that if the necessary infrastructure wasn’t in place by the end of the year, “consumers in the UK will see significant disruption, particularly in the availability of fresh fruit and vegetables”.
Andrew Opie, the BRC’s director of food and sustainability, said: “Food might actually have to be held at the port to be checked. So, you’ve got enormous bureaucracy, enormous change, but crucially you’ve got a problem with the infrastructure at the key ports around the Channel, which currently really act as an extension of the motorway for our supply chain, where you will be holding thousands of vehicles every day.”
EU trade will not be waved through with zero checks, which had been the case under a no-deal Brexit.
Traders will not be able to use special arrangements to lodge new paperwork after a grace period at a later date.
Industries from car manufacturers to food distributors, which rely on the frictionless free flow of goods with the continent, say they face extra costs, delays and red tape from what are known as non-tariff barriers.
Products of animal origin will need export certificates from a registered vet.