The government has told businesses frictionless trade with the EU will end this year with the introduction of import checks at the UK border.
EU trade will not be waved through with zero checks which had been the plan 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.
Officials said firms will have enough notice to prepare for changes in time for 1 January.
Cabinet Minister Michael Gove told attendees at 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."
