Boris Johnson has warned against a potential EU vaccine export "blockade", after the European Commission confirmed plans to tighten rules surrounding supplies to countries outside the bloc.
However, in a sign that the UK and the EU are seeking to defuse an ongoing row, both sides later released a joint statement saying they were seeking a "win-win situation".
The British prime minister was questioned by MPs on Wednesday on the Commission's proposals.
“The partnership we have with our European colleagues is very, very important and we continue to work with them,” Johnson said. “I don’t think that blockades of either vaccines … or ingredients for vaccines are sensible.”
“I would just gently point out to anyone considering a blockade … that companies may look at such actions and draw conclusions about whether or not it is sensible to make future investments in countries where blockades are imposed,” he added.
The Commission's move follows weeks of shortages and delays, especially related to Anglo-Swedish company AstraZeneca, causing great frustration across the continent.
Boris Johnson has sought to ease the tensions over vaccines, speaking by phone in the past few days to European leaders including Commission President Ursula von der Leyen and French President Emmanuel Macron.
The joint statement issued by the EU and the UK said both sides were looking to cooperate and resolve issues over vaccine supplies.
"We are all facing the same pandemic and the third wave makes cooperation between the EU and UK even more important," the statement said, adding that discussions would continue as both sides sought "a reciprocally beneficial relationship" on COVID-19.
"Given our interdependencies, we are working on specific steps we can take - in the short-, medium - and long term - to 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," the statement concluded.
Under the Commission's plan, the principles of "proportionality and reciprocity" will be incorporated into the transparency mechanism that was introduced at the end of January and will assess "case by case" the export requests from pharmaceutical companies.
This means vaccination-leading countries like the UK could face a harder task getting vaccines from EU countries.
The Commission says the EU is the only vaccine producer of the OECD exporting to countries that have production capacities of their own, without enjoying reciprocity in return.
The proposed updated mechanism will be put to EU leaders who are due to meet via videoconference in the European Council on Thursday.