The EU has reversed its decision to trigger an emergency provision in the Brexit deal to control Covid vaccine exports from the EU.
The move could have seen checks at the border of Ireland and Northern Ireland to prevent shipments entering the UK.
The Brussels U-turn came hours after it was announced, following condemnation from London, Dublin and Belfast.
The plans had been part of the EU's new export controls on vaccines, to combat delivery shortfalls.
It is the latest development in a deepening dispute over delays to the production and distribution of Covid vaccines across the EU.
The Brexit deal guarantees an open border between the EU and Northern Ireland, with no controls on exported products.
However, Article 16 of the Northern Ireland Protocol part of the deal allows the EU and UK to choose to suspend any aspects they consider are causing "economic, societal or environmental difficulties".
On Friday evening the EU announced it would trigger the clause and introduce the export controls on its vaccines entering Northern Ireland in a bid to prevent the region becoming a backdoor for jabs to be sent to the UK mainland.
It said the actions were "justified" to avert problems caused by a lack of supply.
But the proposals sparked concern from Irish Prime Minister Micheál Martin as well as all five parties in Northern Ireland's devolved government.
Northern Ireland First Minister Arlene Foster described it as "an absolutely incredible act of hostility" that created a hard Irish border, while Ireland's Foreign Minister Simon Coveney said in a tweet: "The [Northern Ireland] Protocol is not something to be tampered with lightly, it's an essential, hard-won compromise, protecting peace and trade for many."
Later on Friday evening, following talks with UK Prime Minister Boris Johnson, European Commission President Ursula von der Leyen tweeted that the UK and EU had "agreed on the principle that there should not be restrictions on the export of vaccines by companies where they are fulfilling contractual responsibilities".
She said the backtrack, which was outlined in a European Commission statement, came after "constructive talks" with Mr Johnson in which he had expressed "grave concerns" about the initial plan.
The BBC's Brussels correspondent, Kevin Connelly, said Ms von der Leyen's midnight tweet seemed to "wind down" the idea that there could be a "vaccine war" where jab supplies are prevented from reaching the UK.
"Mistake," "misjudgement," "blunder."
These are just some of words EU insiders have been using privately to describe the European Commission's initial decision on Friday to suspend areas of the Brexit deal dealing with Northern Ireland, a part of its Covid vaccine row.
Although it then U-turned on those plans, critics say the damage was already done.
Brussels previously lectured the UK government about respecting the Irish Protocol - which was painfully and carefully drafted during Brexit negotiations.
Now the EU seemed quick to undermine the agreement.
Member state Ireland felt stung that it hadn't been consulted. This all adds to the impression of chaos surrounding the EU's vaccine rollout.
Brussels was already under fire from a growing number of EU countries for having been slow to sign vaccine contracts with pharmaceutical companies.
This "mishap" over the Irish Protocol, as Spain's Foreign Minister called it, hasn't exactly helped the commission's reputation.
Julian Smith, Conservative MP and former Northern Ireland secretary, said the EU had "pulled the emergency cord" without following the proper processes that had been agreed over years of negotiations.
He told BBC Radio 4's Today programme the move came "without anywhere near the level of understanding of the Good Friday Agreement, of the sensitivities of the situation in Northern Ireland".
"It was an almost Trumpian act - I'm very pleased that they've changed their minds," he said, adding that the EU's "major issue" with its Covid vaccine rollout "does not excuse" their actions.
BBC business correspondent Katie Prescott said firms in Northern Ireland were stunned that the European Commission could trigger a change that was only meant for the "most egregious circumstances" - just one month after the UK's post-Brexit transition period with the EU ended.
Conservative MP and former international trade secretary, Dr Liam Fox, said the EU had shown it was "willing to impose [a] hard border on Northern Ireland" despite the "precious" agreements in place to prevent such a situation, with "very little requirement and no notice".
"It was like an episode of Carry On at the [European] Commission yesterday, except that it wasn't funny," he told BBC News.
Mr Martin welcomed the EU's reversal, describing it as a "positive development given the many challenges we face in tackling Covid-19".
It was not thought that the move would have directly disadvantaged Northern Ireland, which gets its vaccine supplies through the UK procurement system.
The Department of Health confirmed Health Secretary Matt Hancock had a "constructive discussion" on the supply of vaccines with his Northern Ireland counterpart Robin Swann, who tweeted it was "essential" confidence and trust were rebuilt after the EU's actions.
The Irish government and its diplomats have spent a huge amount of time and energy educating their EU partners on the nuances of the Irish border.
During the Brexit negotiations there was a slick operation where visiting dignitaries and camera crews were whisked up the road from Dublin airport to see the border in County Louth.
This paid dividends for Ireland: its concerns remained top of the EU's priorities during the negotiations.
Some of the European Commission's most talented officials were deeply involved in the Irish elements of the Brexit deals.
But something clearly went badly awry on Friday.
The use of the Article 16 mechanism, to override part of the Brexit deal, caught the Irish government by surprise.
Those Commission officials with special knowledge of Ireland were either not consulted or not listened to.
Dick Roche, Ireland's former Europe minister, told the BBC the government had been "blindsided" and "embarrassed".
He said the Taoiseach had been "very annoyed" and made "very strong representation" to urge the Commission to change its mind.
Despite later backtracking on Article 16, the EU is still introducing new controls giving its member states the power - should they want to - to block exports of the coronavirus vaccine to countries including the UK, if the company making them has not honoured existing contracts with the EU.
The European Commission said the temporary mechanism is to combat "the current lack of transparency" over vaccine exports outside the EU, and is not an export ban.
But the World Health Organization is among those criticising the move, saying "vaccine nationalism" could prolong the pandemic and further widen global inequality.
Last summer the EU agreed to buy up to 400m doses of the Oxford-AstraZeneca vaccine, and on Friday the EU's drugs regulator approved the vaccine's use for all adults.
But the firm said that due to problems at one of its EU factories, supplies would be reduced by about 60% in the first quarter of 2021.
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