The EU has sparked fury by triggering part of the Brexit agreement to prevent the unfettered flow of Covid-19 vaccines to the UK via Northern Ireland.
The bloc has invoked part of the Northern Ireland Protocol in an attempt to stop vaccines made in the EU from getting into the UK through the back door.
The protocol, which is part of the Brexit deal, allows goods from the EU to be exported to Northern Ireland without checks.
However the EU triggered Article 16 of the protocol tonight to slap temporary export controls on vaccines sent to Northern Ireland.
Northern Ireland's First Minister Arlene Foster described the move as an "incredible act of hostility" and said she has tonight spoken to Boris Johnson and Cabinet Office Minister Michael Gove on the issue.
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Irish premier Micheal Martin spoke to European Commission President Ursula Von Der Leyen to express his concerns, his spokesman said.
Downing Street has said it would not expect its "friend and ally" EU to disrupt its contracts with vaccine suppliers.
A spokesperson for Number 10 said tonight: "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 U.K. 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 U.K. 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."
Meanwhile the Archbishop of Canterbury, Justin Welby, tonight accused the EU, which he said was inspired by Christian social teaching, of "undercutting its basic ethics".
He tweeted tonight: "The European Union was originally inspired by Christian social teaching - at the heart of which is solidarity.
"Seeking to control the export of vaccines undercuts the EU’s basic ethics. They need to work together with others."
The European Union has made the move to prevent Northern Ireland from being used as a back door to funnel coronavirus vaccine from the bloc into the rest of the UK.
Ms Foster called for a "robust response" from the UK Government.
The protocol, with is part of the Brexit withdrawal deal, normally allows for free movement of goods from the EU into Northern Ireland.
Under the terms of the protocol, goods should be able to move freely between the EU and Northern Ireland as the region remains in the single market for goods and still operates under EU customs rules.
The EU has triggered Article 16 of the protocol to temporarily place export controls on this movement in respect of vaccines.
It comes amid a deepening row over the allocation of the Oxford/AstraZeneca vaccine after the company announced delays to its EU operations.
The move to activate Article 16 will frustrate any effort to use Northern Ireland as a back door to bring vaccines into Great Britain.
Mrs Foster said: "By triggering Article 16 in this manner, the European Union has once again 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 Northern Ireland and the Republic of Ireland over the supply chain of the coronavirus vaccine."
The regulation means Northern Ireland will be considered an export territory for the purposes of vaccine sent from the EU/the Republic of Ireland.
Northern Ireland's vaccines arrive from the rest of the UK at present so those will be unaffected.
The DUP leader added: "With the European Union using Article 16 in such an aggressive and most shameful way, it is now time for our Government to step up.
"I will be urging the Prime Minister to act and use robust measures including Article 16 to advance the interests of Northern Ireland and the rest of the United Kingdom."
The DUP has previously pressed the British Government to invoke the Article 16 mechanism because of disruption to the movement of goods between Britain and Northern Ireland.
Cabinet Office minister Michael Gove has told the EU the UK is "carefully considering" the next steps after the bloc triggered an aspect of the Northern Ireland Protocol as part of its export controls on coronavirus vaccines.
The Chancellor of the Duchy of Lancaster spoke to his counterpart on the UK-EU joint committee on Friday, No 10 said.
In a statement, a spokesman said: "CDL Michael Gove just spoke to European Commission Vice President 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."
The European Commission said: "Exports of goods from Northern Ireland to other parts of the United Kingdom cannot be restricted by Union law unless this is strictly required by international obligations of the Union.
"Therefore, movements of goods covered by this regulation between the Union and Northern Ireland should be treated as exports.
"Whilst quantitative restrictions on exports are prohibited between the Union and Northern Ireland, in accordance with Article 5 (5) of the Protocol on Ireland/Northern Ireland, this is justified as a safeguard measure pursuant to Article 16 of that Protocol in order to avert serious societal difficulties due to a lack of supply threatening to disturb the orderly implementation of the vaccination campaigns in the member states."
UUP leader Steve Aiken said Northern Ireland Secretary Brandon Lewis should be embarrassed.
He added: "The EU is unilaterally invoking Article 16 to protect its own interests and it's about time the UK Government did the same instead of being lead actors in a ridiculous charade that there is no border in the Irish Sea and that Article 16 can't be invoked."
Traditional Unionist Voice leader Jim Allister said the EU was showing its "callous" true colours.
He said: "This afternoon's invocation of Article 16 of the protocol to inhibit exports of Covid vaccines from the EU to Northern Ireland is the most telling illustration imaginable that for the EU the protocol is a plaything to be exploited when it suits its selfish interests.
"The idea that the EU cares anything for Northern Ireland or its people is exposed as utterly bogus.
"The effect of today's regulation is to disavow the much vaunted free trade from the EU to Northern Ireland as part of its single market and instead to treat deliveries of vaccines from the EU to Northern Ireland as 'exports', which they can then restrict, lest such vaccines would make their way to Great Britain."
