The European Union briefly triggered a key article of the Brexit treaty on Friday during an escalating row with pharmaceutical giant AstraZeneca over the production and distribution of a COVID-19 vaccine.

Due to supply issues – which have led to the unusual release of the redacted contract between the bloc and AstraZeneca – the European Commission said it would override the Brexit "Trade and Cooperation Deal" with the UK. 

It wants to limit vaccine access to Northern Ireland, to stop it becoming an export route out of the Republic of Ireland, which is an EU member state. 

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The deal, signed on Christmas Eve 2020, guarantees no checks on the Irish-UK border, but Article 16 allows the EU or the UK to "unilaterally take appropriate safeguard measures" if the treaty's application leads to "serious economic, societal or environmental difficulties that are liable to persist." In other words, the EU would have installed border checks on the historically sensitive boundary between Northern Ireland and the Republic. 

The move provoked immediate anger from London, Belfast and Dublin. UK Prime Minister Boris Johnson expressed his "grave concerns" while Northern Ireland First Minister Arlene Foster decried an "incredible act of hostility" that would reintroduce a "hard border" on the island.

The EU backtracked and said it was "not triggering the safeguard clause," claiming it had been an "oversight" – a development which Republic of Ireland leader Micheal Martin described as a "welcome decision" and a "positive development." However, the bloc has not backed down on introducing vaccine export controls from its members, saying it has "no choice."