The first doses are set to be administered on Tuesday, with the National Health Service (NHS) giving top priority to vaccinating the over-80s, frontline healthcare workers and care home staff and residents.

Britain gave emergency use approval for the vaccine developed by Pfizer and BioNTech last week - jumping ahead in the global race to begin the most crucial mass inoculation programme in history.

In total, Britain has ordered 40 million doses. As each person requires two doses, that is enough to vaccinate 20 million people in the country of 67 million.

About 800,000 doses are expected to be available within the first week.

Initial doses that have arrived from Belgium are being stored in secure locations across the country, where they will be quality checked, the health ministry said.

The rollout coincides with a crucial and perilous moment in negotiations between Britain and the European Union on a post-Brexit trade agreement.

A status quo transition period will end on Dec. 31 and a no-deal scenario would lead to major disruptions in the movement of goods between Britain and EU countries such as Belgium.

The Observer newspaper reported on Sunday that, under UK government contingency plans, tens of millions of vaccine doses could be flown to Britain by military aircraft to avoid delays at ports caused by Brexit.

The head of the medicines regulatory agency that approved the vaccine, June Raine, was asked whether she was concerned that a no-deal risked disrupting the rollout.

“We’ve practised, we are ready, we are fully prepared for any possible outcome,” Raine said in an interview on BBC television.