Johnson & Johnson said its Janssen Pharmaceutica unit will supply the UK government with its candidate known as Ad26.COV2.S with an initial sale of 30 million doses on a not-for-profit basis for emergency pandemic use.
The advance purchase agreement will also provide an option for an additional purchase of up to a 22 million doses, it said.
In a separate statement, Novavax said the UK would buy 60 million doses of its coronavirus vaccine candidate, NVX-CoV2373, for a phase 3 clinical trial.
With six deals each so far, Britain and the US are leading the global race to strike deals with drugmakers for vaccines as the pandemic continues to rage.
The latest agreements bring the UK's total number of doses secured to 362 million for the population of 66 million.
J&J said it has also agreed to collaborate with the UK government on a global Phase 3 trial to explore the two-dose regimen of its vaccine candidate.
It will run parallel to the Phase 3 trial investigating the single-dose programme.
No vaccine has yet proven to work, but more than 20 candidates are in clinical trials.
Taylor Jones, Vinit Thakkar and Kyran Jones join forces to launch THG Indiagetty images
Taylor Jones and Vinit Thakkar come together to launch THG India getty images