The vaccine candidate, developed by Sanofi in partnership with GSK, is based on the recombinant protein-based technology used by the former to produce an influenza vaccine and the latter's established pandemic adjuvant technology.
But most vaccines fail in development stages, and Sanofi believes its experience with flu vaccines could be an advantage.
The early access agreement follows another deal the government has made with three pharmaceutical companies to receive up to 90 million doses of potential vaccine candidates.
The deals involve 30 million doses of a vaccine being developed by U.S. pharma giant Pfizer and Germany's BioNTech, and 60 million doses of another created by France's Valneva.
"As expected, our performance this quarter was disrupted by COVID-19, particularly in our Vaccines business, as visits to healthcare professionals were limited due to lockdown measures", said GSK's chief executive Emma Walmsley.
With the latest announcement, the government said it has now secured early access to four different types of immunisation and a total of 250 million doses, giving the United Kingdom the "most likely chance of finding a safe and effective vaccine at the quickest speed".
Sanofi and GSK said talks to supply their vaccine to the EU, Italy and France were ongoing.
The UK is racing ahead, along with the USA and European Union to secure doses of potential vaccines across different vaccine candidates.
The Government has stressed the importance of investing in a broad range of vaccines, to improve the chances of a successful outcome. Depending on whether the data are positive, the companies hope to gain regulatory approval by the first half on 2021. While this agreement is very good news, we mustn't be complacent or over optimistic.
The deals include vaccines being developed by pharmaceutical giants BioNtech and Pfizer as well as the firm Valneva. "We thank the UK Government for confirmation of purchasing intent, which supports the significant investment we are already making as a company to scale up development and production of this vaccine", Roger Connor, president of GSK vaccines said in a statement.
The University of Oxford's partner, AstraZeneca Plc, intends to make as many as 30 million doses available to the U.K.by September as part of a pact to deliver 100 million doses.
The companies plan to provide a significant portion of total worldwide available supply capacity to "Access to COVID‐19 Tools (ACT) Accelerator", a global initiative tasked with providing equitable access to COVID-19 tests, treatments, and vaccines.