There was chaos in Brussels on Friday night after the EU Commission was forced to backtrack over plans to introduce a "hard vaccine border" in Ireland as part of a no-warning bid to ban EU vaccine exports to the UK.
The move followed hours of diplomatic chaos after it emerged the EU triggered an article of the Protocol which would have enabled the EU to place checks on goods entering Northern Ireland from the Republic to ensure no EU-manufactured vaccines were present.
The Brussels source said a "mistake was made somewhere along the way" and insisted EU vaccines will not be stopped from entering Northern Ireland.
The EU was concerned Northern Ireland could be a "back door" for EU vaccines into the rest of the UK - in breach of the EU ban on exports.
Under the Brexit deal agreed between the UK and EU goods are permitted to move freely between the North and South of Ireland. Article 16 of the Northern Ireland Protocol allows either side introduce controls on goods in emergency situation.
The shock EU manoeuvre sparked concerted outrage from political leaders in Belfast, London and Dublin, forcing the Brussels to stage a climbdown, details of which remained sketchy as we went to press.
It was not clear whether the commission would withdraw the entire export ban on EU vaccines, or was planning only to tweak its use of the controversial Northern Ireland Protocol which has hit trade between Great Britain and Northern Ireland.
First Minister Arlene Foster described the EU plans as "an incredible act of hostility".
The DUP leader, speaking after an official document published in Brussels, said the EU would invoke Article 16 of the Northern Protocol as a "safeguarding measure" in order to include Northern Ireland in the planned export ban.
The EU plan would create a "hard vaccine border" in Ireland, and its use of the Article 16 mechanism of the NI protocol will increase pressure on the UK Government to do the same in order to reduce the increasing disruption to GB-NI trade caused by the Irish Sea Border.
Mrs Foster said: "By triggering Article 16 in this manner, the EU has once 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 NI and the Republic of Ireland over the supply chain of the coronavirus vaccine."
Describing the EU action as "aggressive and shameful" Mrs Foster added: "It is now time for our Government to step up.
"I will be urging the Prime Minister to act and to use robust measures - including Article 16 - to advance the interests of the Northern Ireland and the rest of the United Kingdom," she said.
Neither the Irish nor the UK Governments were warned in advance of the EU's export ban plan .
In the wake of the shock EU move, the UK Government warned Brussels it was "carefully considering" its next steps.
Chancellor of the Duchy of Lancaster Michael Gove 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".
And a terse statement from Downing Street said: "The UK Government is urgently seeking an explanation from the European Commission about the statements issued by the EU today and assurances as to its intentions.
"The UK has legally-binding agreements with vaccine suppliers and it would not expect the EU, as a friend and ally, to do anything to disrupt the fulfilment of these contracts.
"The UK government has reiterated the importance of preserving the benefits of the Belfast/Good Friday agreement and the commitments that have been made to the two communities'.
The chairman of Westminster's influential NI Affairs Committee, Simon Hoare, also hit out at the EU's unilateral action.
He said it was "unconscionable folly" for the EU to escalate its vaccines row by triggering the protocol, adding: "We need calm, stability and level-headedness."
A spokesperson for Micheál Martin told RTE News that the Taoiseach was aware of the issue and is "in discussions with Ursula von der Leyen to raise concerns about this", while Dublin Foreign Affairs Minister Simon Coveney tweeted: "We are working with the EU Commission to try to resolve this issue and protect the integrity and operation of the NI Protocol."
The UK is the only European country to be subject to the EU's vaccine export ban plan, which would block UK access to other vaccines, including the Pfizer jab, which is made in Belgium.
In Northern Ireland, politicians from every party criticised the EU's actions.
Sinn Fein's Michelle O'Neill. Stormont's deputy First Minister, said she had "very serious concerns" over the EU actions.
"This is a totally ill-judged move by the EU and should not have been triggered," she said.
"Calm heads need to prevail, this needs sorted urgently."
Ulster Unionist leader Steve Aiken called on the UK Government to act to protect Northern Ireland's interests: "For weeks now we have been told that Article 16 cannot be invoked accompanied by all sorts of feeble excuses for not doing so," he said.
"Yet the EU has done it at the drop of a hat," he said.
Alliance party North Down MP Stephen Farry - who had supported the NI Protocol - also criticised the EU action. "Use of Article 16 is NOT justified or warranted in this situation," he tweeted.
SDLP leader Colum Eastwood said on social media: "Invoking Article 16 is disproportionate and a grave error in judgment by the European Commission. We face a common threat. Our response should be characterised by our common values - co-operation & solidarity."
The EU move comes as First Minister Arlene Foster met representatives of vaccine producer AstraZeneca who, she said, raised supply chain issues for Northern Ireland in terms of getting the vaccine here next year.
Earlier UUP leader Steve Aiken appealed for calm amid rising loyalist tensions over the Brexit settlement.