The company partnered with Oxford University to deliver a vaccine, which is now the source of frustrations within the EU after AstraZeneca warned it would not be able to deliver as many jabs in the first quarter of the year as originally promised.
"UK factories are part of our advanced purchase agreement and that is why they have to deliver," EU Health Commissioner Stella Kyriakides told a press conference on Wednesday.
'AstraZeneca needs to deliver on its commitments in our agreements'
AstraZeneca pulled out of a meeting with the bloc to discuss delayed vaccine commitments on Wednesday, according to an EU official.
But in an interview with Italian newspaper La Repubblica, AstraZeneca boss Pascal Soriot said the UK had experienced the very same "teething issues" now experienced by the EU.
"We've also had teething issues like this in the UK supply chain. But the UK contract was signed three months before the European vaccine deal," he said.
"So with the UK we have had an extra three months to fix all the glitches we experienced. As for Europe, we are three months behind in fixing those glitches."
The EU has threatened to impose tight controls on the export of the jabs after AstraZeneca announced a delay - and the issue was exacerbated after Pfizer - another vaccine manufacturer - also announced delivery holdups.
Mr Soriot said on Tuesday: "In the EU agreement it is mentioned that the manufacturing sites in the UK were an option for Europe, but only later."
ITV News Science Editor Tom Clarke explains why the UK's supply should not be impacted for now
He added: "The UK agreement was reached in June, three months before the European one.
"As you could imagine, the UK government said the supply coming out of the UK supply chain would go to the UK first. Basically, that's how it is."
But EU Commission officials have said the company also committed to providing vaccines from two factories in Britain.
Officials added that AstraZeneca had not provided sufficient explanations on why doses could not be shipped from factories with no production problems - like those in the UK.
Speaking in the House of Commons on Wednesday, the prime minister claimed the UK had been able to do things "better in some ways" with regards to the vaccine programme.
Asked if Brexit had helped speed up the UK's vaccine roll out, Boris Johnson said: "We certainly were able to use speed and agility to deliver on the programme that we needed to do.
"It's early days and it’s very, very important to remember that this is an international venture, these vaccines, we depend on our friends and partners and we will continue to work with those friends and partners in the EU and beyond."