The European Union on Wednesday unveiled its plan to tighten vaccine export control rules to prevent what it sees as a one-way flow of doses leaving the bloc.
The plan, which will go before an EU leaders' summit on Thursday, would limit vaccine exports to countries such as Britain that produce some vaccines but do not send any to the bloc.
It stops short of a full export ban but any shipment of doses due to leave the EU will be assessed on the destination country's inoculation rate and vaccine exports.
The proposal came as millions of AstraZeneca doses were reportedly found in Italy.
Brussels directed an Italian government inspection of the Anagni factory, south-east of Rome, where AstraZeneca vaccines are bottled. The raid uncovered 29 million doses that authorities hadn't recorded.
The EU is embroiled in a months-long dispute with AstraZeneca over a shortage of deliveries to the continent.
Europe will now insist on reciprocity as vaccination rates in Britain rise but the continent's inoculations proceed slowly. The bloc said it wants two AstraZeneca plants in the UK to deliver doses to EU countries.
The UK government is understood to be willing to share supplies from the Netherlands-based Halix plant with the EU.
In a joint statement on Wednesday, the UK and the EU agreed to cooperate on vaccine supply as discussions continued.
"We are all facing the same pandemic and the third wave makes cooperation between the EU and UK even more important," the statement said.
"We have been discussing what more we can do to ensure a reciprocally beneficial relationship between the UK and EU on Covid-19.
"Given our interdependencies, we are working on specific steps we can create a win-win situation and expand vaccine supply for all our citizens.
"In the end, openness and global cooperation of all countries will be key to finally overcome this pandemic and ensure better preparation for meeting future challenges."
President of the European Commission Ursula von der Leyen said the bloc exported vaccines to dozens of countries but "open roads should run in both directions".
"This is why the European Commission will introduce the principles of reciprocity and proportionality into the EU's existing authorisation mechanism," she said.
"The EU has an excellent portfolio of different vaccines and we have secured more than enough doses for the entire population. But we have to ensure timely and sufficient vaccine deliveries to EU citizens. Every day counts."
Ms von der Leyen said the EU approved the export of some 41 million vaccine doses to 33 countries in the last six weeks and believes the bloc is at forefront of international vaccine-sharing efforts.
Under the less stringent export control system already in force, only one shipment in 381 has been barred.
UK Prime Minister Boris Johnson on Tuesday told a private meeting of MPs that capitalism and greed were the reasons for the success of Britain’s Covid-19 vaccination drive.
He also praised pharmaceutical companies for developing the vaccines in record time.
But his more controversial remarks could provide ammunition for the EU countries embroiled in a bitter row with drug maker AstraZeneca over a shortfall in supplies.
Mr Johnson's comments, reported by The Sun newspaper, were made during a Zoom meeting with Conservative Party MPs.
"The reason we have the vaccine success is because of capitalism, because of greed, my friends," Mr Johnson said.
He later said "I regret saying it" and repeatedly asked MPs to "forget I said that".
It is understood the prime minister did not compare the UK's vaccination campaign with those of EU countries during his address.
The Downing Street office declined to comment on the reports.
Home Secretary Priti Patel said she was not in the meeting.
"The prime minister always acknowledges the strong success that we've had in terms of the vaccine, not just the roll-out, which is incredible, but also our ability as a country to develop the vaccine, and the role that pharmaceutical companies and science and technology has played in that," she said.
Britain is gradually easing lockdown restrictions in a plan underpinned by the success of the national vaccination campaign, under which more than 28 million people received a first vaccine dose.
The UK stands to lose the most under the new European rules, as the biggest benefactor of EU exports so far.
Last week, Ms von der Leyen demanded “reciprocity” from Britain after she said the EU had sent 10 million vaccines across the Channel over the past six weeks, mostly from Belgium’s Pfizer plant, but the UK had exported none to the bloc.
German Chancellor Angela Merkel on Tuesday threw her weight behind Ms von der Leyen's targeted ban but ruled out an outright ban.
"We have a problem with AstraZeneca," she said.
European rifts on the issue were exposed on Sunday when Irish Prime Minister Micheal Martin said the EU’s threats would be a retrograde step and risked retaliation from Britain.
Philippe Lamberts, a Belgian Member of the European Parliament, said he hopes the EU does not block supplies to Britain.
"The quarrel is not between the European Commission and the British government, it is between the Europeans and one of their suppliers who have a track record of dishonesty," he told BBC Radio 4's Today programme on Wednesday.
He said AstraZeneca "overpromised and underdelivered" vaccines to the bloc.
He described the Anglo-Swedish drug maker as "a company that cannot be relied upon" and said other pharmaceutical companies had "engaged constructively" with the EU.
Ms Merkel's intervention came as she announced now-reversed plans for a tougher lockdown in Germany over the Easter holidays. All shops were to close between April 1 and April 5, with only grocers allowed to open on Saturday, April 3.
The plan was unexpectedly dropped on Wednesday after she admitted it was not practical.
“This mistake is my mistake alone. I deeply regret this and ask all citizens for forgiveness," Ms Merkel said.
Germany is fighting the same battle to contain a third wave of infections as neighbouring France.
Medical workers warned French hospitals face a "violent shock" if the surge in cases is not contained.
