Sept 1 (Reuters) - AstraZeneca has expanded its previous agreement with Oxford Biomedica to mass-produce the British drugmaker's COVID-19 vaccine candidate, as it looks to scale-up supply ahead of a possible fast-track approval from the United States.

Gene and cell therapy firm Oxford Biomedica said on Tuesday AstraZeneca would give it 15 million pounds ($20 million) upfront to reserve manufacturing capacity at its plant and that it could get an additional 35 million pounds plus other costs until the end of 2021 under the new 18-month deal.

AstraZeneca's vaccine hopeful for the novel coronavirus, AZD1222, already among the leading candidates in the global race for a successful vaccine, moved to late stage trials in the United States this week as the company targets 3 billion doses of the vaccine.

Oxford Biomedica was among AstraZeneca's initial partners when they teamed up in May to produce the vaccine and Tuesday's deal could be further expanded by another year and a half into 2022 and 2023, Oxford said.

($1 = 0.7458 pounds)

(Reporting by Pushkala Aripaka in Bengaluru; editing by Patrick Graham)