A political row erupted after the EU moved to use the protocol to stop the unimpeded flow of vaccines from the bloc into Northern Ireland.
The European Union has since backed down on its move to override part of the Brexit deal on Northern Ireland – but what is Article 16 of the Northern Ireland protocol?
Here’s what you need to know.
What is Article 16 NI protocol?
Under Brexit’s Northern Ireland protocol, all products are normally permitted to be exported from the EU to Northern Ireland without checks.
The protocol was negotiated at the 11th hour as a solution to the Irish Border, one of the biggest sticking points in post-Brexit trade talks. Under previous proposals in the talks, there were rows over the functioning of the sole land border between the EU and UK, which is on the island of Ireland.
Rather than the introduction of a militarised border in Ireland, it was agreed that Northern Ireland would effectively remain in the single market for goods.
But what does this mean? Effectively it means EU customs rules are applied at Northern Ireland’s ports and airports, even though the region is still part of the UK customs territory.
Article 16 Northern Ireland: EU ends threat to override NI Brexit deal in bid to control vaccine exports
Article 16 of the protocol gives the EU or the UK the ability to unilaterally suspend aspects of its operation if either side considers that aspect is causing “economic, societal or environmental difficulties”.
The protocol theoretically presents a back door for exporters to circumvent those controls and move goods into Great Britain unfettered, because trade from the EU into Northern Ireland is unrestricted under the protocol, as is trade from Northern Ireland into other parts of the UK.
When can Article 16 be triggered?
Article 16 is only supposed to be triggered in the face of “serious” problems, and if one side triggers Article 16, the other side can take rebalancing action in response.
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What are the thoughts of politicians on Article 16?
The protocol is regarded with hostility by unionists as a border down the Irish Sea, with some DUP MPs calling for Article 16 to be triggered over delays faced by hauliers transporting goods from Great Britain to Northern Ireland.
There are also claims that Brexit red tape could hinder the movement of military equipment within the UK.