Professor Sarah Gilbert told BBC Radio 4's Today programme that clinical trials of the vaccine would "be starting quite soon" and that production processes need to be set in motion quickly.

"We need to start manufacturing large amounts of the vaccine," she said.

"It is not uncommon for companies to start manufacturing a new vaccine before they really know for certain it works. No-one wants to be in a position where you have a vaccine which you have shown does work and then not have any of the vaccine to use.

"The sooner we start the quicker we're going to get to the billion dose scale.

"That probably won't be this year but if we don't start we're not going to get there next year either."