AstraZeneca's vaccine contract with the UK contains the same clause blamed for the EU's struggling inoculation campaign.
The agreement says the "best reasonable efforts" will be made to supply Britain with 100 million doses.
AstraZeneca said last month that it would reduce vaccine deliveries to the EU because of production problems at its Belgian plant. The company's chief executive, Pascal Soriot, quoted the "best efforts" clause to demonstrate the company was not legally bound to supply doses.
"It's not a commitment we have with Europe, it's a best effort," he told Italian newspaper La Repubblica at the time.
Similar language is included in the UK supply agreement.
CNN obtained a redacted version of the UK's contract through a freedom of information request.
Officials provided the US broadcaster with a link to the contract on a government website, where it was posted in November but appeared to have gone largely unnoticed.
UK ministers refused to publicly release details of the contract for commercial reasons.
The agreement states that AstraZeneca can “update and refine” the delivery schedule when necessary. The company must give the government at least 30 days’ notice before each delivery.
Mr Soriot said last month that the company would supply the UK with doses before European markets because an agreement was reached with Britain first.
However, the UK's contract with AstraZeneca is dated August 28 – one day after the EU contract. The agreement also states that Britain could receive doses manufactured in EU member states.
Asked whether UK priority for doses was explained parts of the contract that were redacted, AstraZeneca told CNN: "The UK government has an agreement with AstraZeneca to supply 100 million doses of the University of Oxford-AstraZeneca vaccine, and has agreed delivery timescales for this.
"The detail of any commercial agreements between the UK government and AstraZeneca for the Oxford-AstraZeneca vaccine are commercially sensitive."
Europe was outraged when AstraZeneca reduced vaccine deliveries to the bloc, threatening to restrict exports to the UK.
Northern Ireland’s First Minister Arlene Foster accused the EU of an “incredible act of hostility” after it threatened to use a clause in the Brexit agreement to restrict vaccine supplies to the province.
The EU retracted the threat and said it was a mistake to invoke the clause. But the bloc pushed ahead with a new “transparency mechanism” that forces pharmaceutical companies to notify authorities about vaccines due to be exported.
European Commissioner for Health and Food Safety Stella Kyriakides said at the time that the bloc deserved its "fair share" of doses and rejected AstraZeneca's argument that delivery should be on a "first come, first served" basis.
"That may work at the neighbourhood butcher's but not in contracts and not in our advanced purchase agreements,” she said.
The UK has given 15.9 million people, about 23 per cent of its 67 million population, their first dose of Covid-19 vaccine, behind only Israel and the UAE in inoculations per capita.
The EU has vaccinated about 5 per cent of its 447 million population, according to the University of Oxford's Our World In Data.