The additional doses will be used alongside other approved vaccines for the booster programme, which the government said on Wednesday is set to begin in the autumn.
Hancock said booster shots were the “best way to keep us safe and free” and “protect the progress that we’ve all made”.
“Our vaccination programme is bringing back our freedom, but the biggest risk to that progress is the risk posed by a new variant,” the health secretary added.
Booster shots are designed to provide an immune system top-up while also offering some extra protection against new variants.
According to the Department for Health and Social Care (DHSC) the UK had given out more than 47.5m vaccine doses as of April 27.
Included in that figure are just shy of 34m people having had their first dose (64.5% of all adults) and 13.6m having had their second (25.8% of all adults).
Ben Osborn, from Pfizer, said the new government order would more than double its current supply commitment to the UK.
“Along with our partner BioNTech, we are working relentlessly to support vaccination campaigns worldwide and, based on current projections, believe we can deliver more than 2.5bn doses of our vaccine globally by the end of 2021,” he said.
It came as Professor Anthony Harnden, deputy chair of the Joint Committee on Vaccination and Immunisation (JCVI), warned people not to “all go completely wild” and ignore social distancing rules.
“We need to celebrate our success with vaccines,” he told MPs on the Commons science and technology committee. “But we also need to be cautious because we don’t want to see what’s happening in other parts of Europe and other parts of the world here in the UK.
“If we can carry on with the messaging that we carry on being cautious, even though we are unlocking slowly in terms of the social distancing, the mask wearing, et cetera, we may keep infection rates down.”