The EU has dropped a controversial plan to prevent vaccines moving from the bloc to Northern Ireland, after the decision provoked fury in London, Dublin and Belfast.
The proposed blockade by the EU prompted the World Health Organisation to warn against "vaccine nationalism" and threatened to harm Brexit relations barely a month after the deal was signed.
British-Swedish firm AstraZeneca has said it can only deliver a fraction of the vaccine doses it promised the EU and Britain because of production problems.
As vaccine production stutters, both the EU and the UK demanded their contracts were met, with the EU threatening to stop vaccine exports into Northern Ireland from Ireland - the EU's only land border with the UK.
But now the EU has backtracked, with EU Commission President Ursula von der Leyen confirming: "We agreed on the principle that there should not be restrictions on the export of vaccines by companies where they are fulfilling contractual responsibilities."
Julian Smith, a former Northern Ireland secretary with the UK government, said UK and Irish leaders had been caught by surprise, when the EU tried to trigger article 16 of the Northern Irish Protocol, a clause devised as a last resort to alleviate serious disruption to trade in Northern Ireland after Brexit.
The now-annulled move had been designed to prevent the open border between EU-member Ireland and Northern Ireland from acting as a backdoor for vaccine supplies into the United Kingdom.
"The EU cocked up big time last night, but we all need to work in the interest of preserving Northern Ireland," Mr Smith said.
"It is not just a backdoor for goods going to Britain, it is a very sensitive place and we have a duty between the EU and UK to ensure there is no hard border."
Irish Foreign Minister Simon Coveney said on Twitter: "Welcome news, but lessons should be learned; the protocol is not something to be tampered with lightly, it's an essential, hard won compromise, protecting peace & trade for many."
WHO chief Tedros Adhanom Ghebreyesus warned that there was a "real danger that the very tools that could help to end the pandemic -- vaccines -- may exacerbate" global inequality.
The EU, whose member states are far behind the UAE, Israel, Britain and the United States in distributing vaccines, is scrambling for supplies as the West's biggest drugmakers slow deliveries to the bloc due to production problems.
By contrast, the United Kingdom has secured 367 million doses of the seven most promising vaccines – including 100 million doses of the AstraZeneca vaccine, developed by Oxford University.
Among those procured by Britain are vaccines developed by Novavax, which on Thursday said it proved 89 per cent effective in a UK trial, and Johnson & Johnson, which on Friday said its shot was between 66 per cent and 72 per cent effective.
EU had justified proposed move on grounds of societal unrest
The British-Swedish drugmaker AstraZeneca was caught in the crosshairs after it said last week it would fall short of delivering promised vaccines to the EU by March, because of production problems in Belgium.
That has angered Brussels which has demanded to know why it cannot divert supplies from its British sites, which have been producing millions of shots for British citizens.
The European Commission has agreed to a plan to control exports of vaccines from the bloc, including to Britain, arguing it needs to do so to ensure its own supplies.
Before reversing the decision to invoke Article 16, the commission had justified its move to "avert serious societal difficulties" in EU states due to a lack of vaccines.
Britain has its own domestic supply chain in place for AstraZeneca's shot, including distribution in Northern Ireland, but it imports the Pfizer-BioNTech vaccine from a factory in Belgium.