The row between the European Union and the United Kingdom over the AstraZeneca vaccine is again heating up, with European Commission chief Ursula von der Leyen warning that the bloc could stop its exports from the EU.

“We have the option of prohibiting a planned export,” von der Leyen said in an interview with Germany’s Funke Mediengruppe over the weekend.

That is the message to AstraZeneca: You first fulfill your contract with Europe before you start delivering to other countries,” she added.

For background: The latest comments come after the pharmaceutical giant announced another shortfall in expected vaccine deliveries to the EU last week. Von der Leyen slammed AstraZeneca for alleged under-production and under-delivery, blaming them in part for the slow vaccine rollout in Europe.

Multiple UK media on Monday morning reported that Prime Minister Boris Johnson will be contacting individual EU leaders to urge them not to support the idea of a block on vaccine exports ahead of an EU leaders’ meeting Thursday, although Downing Street declined to comment when contacted by CNN.

Asked on Sunday about the risk of the EU blocking vaccine exports to the UK, British Defense Secretary Ben Wallace warned the European Commission that the “world is watching what happens.”

“The European Union will know that the rest of the world is looking at the Commission about how it conducts itself on this and if, if, if contracts get broken and undertaken, you know that that is a very damaging thing to happen for a trading bloc that prides itself on the rule of law, prides itself on following you know contracts being an open trading bloc,” Wallace told Sky News.