More checks will be needed on some goods entering Northern Ireland from Britain, the British government has confirmed.
It is contained in the Irish protocol of the Brexit Withdrawal Agreement.
That was designed to avoid the emergence of a hard border on the island of Ireland after Britain left the European Union.
The British government has unveiled how it will implement the measure, which takes effect on January 1st, regardless of whether a trade deal is struck with the EU.
Northern Ireland will still follow some EU rules on agricultural and manufactured goods.
Under the move, customs facilities in Northern Ireland would be "expanded".
These would be used to process new electronic import declaration requirements, to check whether businesses are exempt from tariffs because the goods are staying in Northern Ireland or should pay them as the objects are heading on into the EU.
But the UK government is insisting no separate customs infrastructure will be built.
The whole of the UK is set to leave the EU customs union - but Northern Ireland will continue to enforce EU customs codes at its ports.
UK Cabinet Office Minister Michael Gove says businesses in Northern Ireland will benefit from the approach.
"If the protocol is implemented in line with our approach, that means they will have unfettered access to the rest of the UK's internal market and also free access to the EU's single market.
"That is a great prize, and one that I believe all businesses in Northern Ireland would want us to help them to grasp".
A transition period is still in place until the end of 2020, while the UK and EU negotiate arrangements.
The current rules on trade, travel, and business for the UK and EU continue to apply during the transition period.
But new rules will take effect on January 1st 2021.