The UK has ordered 40 million more doses of Valneva's COVID-19 vaccine candidate for 2022, bringing its total order to 100 million.
The French drugmaker said on Monday that Britain could still order an further 90 million doses between 2023 and 2025.
The total value of the entire order was up to 1.4bn euros (£1.24bn) the firm said, adding that it expected a read-out of its Phase I/II clinical trial within three months.
If approved, 60 million doses could start to be delivered to the UK by the second half of 2021, with the remaining 40 million being delivered in 2022.
The jab is expected to be administered in two doses and is being made at a site in West Lothian, with the government claiming the new deal will "bolster long-term vaccine production in Scotland".
Business Secretary Kwasi Kwarteng, said: "This latest deal is yet another weapon in our national arsenal against this terrible disease, and will ensure we have sufficient supplies to protect the British public in 2021 and beyond.
He said Valneva's site in Scotland would be a "vaccine production powerhouse" with the backing of major investment from the UK government.
"Thanks to our incredible UK Vaccine Taskforce, we have now secured a bumper portfolio of over 400 million vaccines, putting our country in an exceptionally strong position to defeat this virus once and for all," he added.
Vaccines minister Nadhim Zahawi said: "If approved, Valneva's vaccine will not only help tackle COVID-19 here in the UK, but aid our mission to ensure there is a fair supply of vaccines across the globe.
"No-one is safe till the whole world is safe."
Health Secretary Matt Hancock added: "If the vaccine is authorised by the health regulator, it will be rolled out across the four nations as quickly as possible."