The Government has warned the European Union it is considering action after the bloc imposed export controls on coronavirus vaccines and impinged on the post-Brexit deal on Northern Ireland.
Cabinet Office minister Michael Gove spoke to the EU on Friday to express concern at its triggering of an aspect of the Northern Ireland Protocol to stop the unimpeded flow of jabs from the bloc into the region.
DUP leader Arlene Foster described Brussels' move as an "incredible act of hostility" that places a "hard border" between the region and the Republic of Ireland, as the EU's row over vaccine shortfalls dramatically deepened.
She demanded a "robust response" from the UK Government and spoke with Prime Minister Boris Johnson and Mr Gove on Friday evening.
The EU took the surprise step of triggering Article 16 of the protocol while embroiled in a row with pharmaceutical giant AstraZeneca over delays in the delivery of jabs.
Mr Gove, the Chancellor of the Duchy of Lancaster (CDL), spoke to his counterpart on the EU-UK joint committee, Maros Sefcovic, to "express the UK's concern over a lack of notification from the EU about its actions in relation to the NI protocol".
"CDL said the UK would now be carefully considering next steps," a statement from Downing Street added.
Shadow Northern Ireland secretary Louise Haigh said the EU's move is "deeply destabilising" and urged the EU to "revoke this action".
"Unilateral actions like this do nothing to aid the stability in Northern Ireland which the protocol was designed to preserve," the Labour MP said.
The protocol, which is part of the Withdrawal Agreement, is designed to allow the free movement of goods from the EU into Northern Ireland, and prevent the need for a hard border on the island of Ireland.
But triggering Article 16 temporarily places export controls on the movement of vaccines, a move taken by the EU to prevent the region being used as a back door to move coronavirus vaccines from the bloc into the UK.
Mrs Foster said: "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."