Stormont First Minister Arlene Foster branded the EU's triggering of Article 16 of Brexit's Northern Ireland Protocol to stop unfettered flow of inoculations from the EU into the region an "incredible act of hostility".
Ms Foster spoke with Prime Minister Boris Johnson and Cabinet Office minister Michael Gove about the issue on Friday evening, and called for a "robust response" from the UK Government.
The Irish premier Micheal Martin is in discussions with European Commission President Ursula Von Der Leyen to express his concerns, according to an Irish government spokesperson.
The protocol, which is part of the Brexit withdrawal deal, normally allows for free movement of goods from the EU into Northern Ireland.
Under the terms of the protocol, goods should be able to move freely between the EU and Northern Ireland as the region remains in the single market for goods and still operates under EU customs rules.
The EU has triggered Article 16 of the protocol to temporarily place export controls on this movement in respect of vaccines.
Responding to the news, PM Boris Johnson said the EU must “urgently clarify its intentions.”
It comes amid a deepening row over the allocation of the Oxford/AstraZeneca vaccine after the company announced delays to its EU operations.
The move to activate Article 16 will frustrate any effort to use Northern Ireland as a back door to bring vaccines into Great Britain.
Mrs Foster said: "By triggering Article 16 in this manner, the European Union has once again shown it is prepared to use Northern Ireland when it suits their interests but in the most despicable manner - over the provision of a vaccine which is designed to save lives.
"At the first opportunity, the EU has placed a hard border between Northern Ireland and the Republic of Ireland over the supply chain of the coronavirus vaccine."
The regulation means Northern Ireland will be considered an export territory for the purposes of vaccine sent from the EU/the Republic of Ireland.
Northern Ireland's vaccines arrive from the rest of the UK at present so those will be unaffected.
The DUP leader added: "With the European Union using Article 16 in such an aggressive and most shameful way, it is now time for our Government to step up.
"I will be urging the Prime Minister to act and use robust measures including Article 16 to advance the interests of Northern Ireland and the rest of the United Kingdom."
The DUP has previously pressed the British Government to invoke the Article 16 mechanism because of disruption to the movement of goods between Britain and Northern Ireland.
Cabinet Office minister Michael Gove has told the EU the UK is "carefully considering" the next steps after the bloc triggered an aspect of the Northern Ireland Protocol as part of its export controls on coronavirus vaccines.
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