Boris Johnson has expressed his "grave concerns" over the EU's move to impose export controls on coronavirus vaccines and impinge on the post-Brexit deal on Northern Ireland.
The Prime Minister held a call with European Commission president Ursula von der Leyen on Friday evening as Downing Street warned the bloc not to disrupt the supply of jabs.
In an extraordinary move that blindsided both the UK and Ireland, the EU invoked Article 16 of the Northern Ireland Protocol to stop the unimpeded flow of jabs from the bloc into the region.
Northern Ireland's First Minister Arlene Foster described Brussels' move as an "incredible act of hostility" that places a "hard border" between the region and the Republic of Ireland.
Brussels took the surprise step that provoked condemnation from across the political spectrum while embroiled in a row with pharmaceutical giant AstraZeneca over shortfalls in the delivery of jabs.
However, as EU Brexit negotiator Michel Barnier urged the bloc to step down from its deepening row over vaccine shortages, EU sources suggested the bloc may U-turn on the move.
A No 10 spokesman said: "The Prime Minister spoke to European Commission president Ursula von der Leyen this evening.
"He expressed his grave concerns about the potential impact which the steps the EU has taken today on vaccine exports could have."
Mr Johnson earlier urged the EU to "urgently clarify its intentions" and detail how it will honour its commitments to the peace process, during a "constructive discussion" with Irish Premier Micheal Martin.
The Prime Minister "set out his concerns" over Brussels' move in a discussion with his Irish counterpart and raised "what these actions may mean for the two communities in Northern Ireland", No 10 said.
A Downing Street statement added: "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."
Senior Cabinet minister 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 warned Britain "would now be carefully considering next steps".
The Taoiseach held multiple calls with Ms von der Leyen, and the PA news agency understands that Mr Martin was not given advance notice of Brussels' decision to invoke the protocol.
The protocol, which is part of the Withdrawal Agreement, is designed to allow the free movement of goods from the EU into Northern Ireland, and prevent the need for a hard border on the island of Ireland.
But triggering Article 16 temporarily places export controls on the movement of vaccines, a move taken by the EU to prevent the region being used as a back door to move coronavirus vaccines from the bloc into the UK.
The European Commission's new regulation states: "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."
In an interview with The Times, Mr Barnier called for a "spirit of co-operation" during the "extraordinarily serious crisis".
"And I believe that we must face this crisis with responsibility, certainly not with the spirit of oneupmanship or unhealthy competition," he added.
It was not immediately clear what steps the Government was considering, but culture minister Caroline Dinenage did not rule out the UK invoking Article 16 in retaliation, as called for by the DUP.
"The stakes are really high and everybody needs to keep their heads about them," she said on BBC Radio 4's Any Questions?, as she assured "we're very confident in our supplies".
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.
— Archbishop of Canterbury (@JustinWelby) January 29, 2021
Archbishop of Canterbury the Most Rev Justin Welby urged the EU to rethink its actions.
"Seeking to control the export of vaccines undercuts the EU's basic ethics. They need to work together with others," he tweeted.
Mrs Foster said: "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."
Northern Ireland Secretary Brandon Lewis said the region receives its vaccines as part of UK procurement.
But preventing vaccines made with the EU from being exported could hinder the UK's access to further supplies, particularly to the Belgian-made Pfizer jab.
Brussels has also demanded doses of the AstraZeneca vaccine manufactured in British plants in order to solve its supply shortage issues, as member states were forced to pause or delay their rollouts.
The EU's "vaccine export transparency mechanism" will be used until the end of March to control vaccine shipments to nations outside the bloc.
It seeks to ensure that any exporting company based in the EU first submits its plans to national authorities.
European Commission executive vice-president Valdis Dombrovskis told a Brussels press conference: "Today the commission has adopted an implementing regulation making the export of certain products subject to an export authorisation.
"This regulation concerns the transparency and export of Covid-19 vaccines."
The UK was not named among countries exempted from the new measures.
Meanwhile, AstraZeneca published a redacted version of its contract with the EU, which the bloc said was important for "accountability".
The contract mentions that the firm would use "best reasonable efforts" to use European plants, including two in the UK, as production sites for vaccines destined for the EU.
The row intensified as the European Medicines Agency (EMA) authorised the AstraZeneca jab, which it developed with Oxford University, for all adults throughout the European Union.