Britain is close to a 500 million pound ($624 million) supply deal with Sanofi and GlaxoSmithKline for 60 million doses of a potential COVID-19 vaccine, the Sunday Times reported.
Clinical trials are due to start in September and Sanofi has said it expects to get approval by the first half of next year, sooner than previously anticipated.
More than 100 vaccines are being developed and tested around the world to stop the COVID-19 pandemic and governments are racing to secure supplies of vaccines even before their efficacy is proven.
A supply agreement with Sanofi and GSK would be Britain's second such deal, after it said it would buy 100 million doses of the Oxford-AstraZeneca vaccine.
Sanofi was not immediately available to comment, while a spokesman for GSK declined to comment. Both have both said they are prioritising quality over speed in developing a vaccine.
"The Government's Vaccines Task Force is actively engaging with a wide range of companies both in the UK and abroad to negotiate access to vaccines," a spokeswoman for Britain's business ministry, said, without confirming if Sanofi or GSK were among them.
The Sunday Times report said that Britain was considering taking an option to buy the vaccine should it work in human trials, and would see the amount paid in stages.
Sanofi is working on two possible COVID-19 vaccines, one of which uses an adjuvant made by GSK to potentially boost its efficacy, and has said it has capacity to produce up to 1 billion doses a year.
Its timeline for clinical trials is behind the likes of Moderna Inc, the University of Oxford in collaboration with AstraZeneca Plc, and an alliance of BioNTech and Pfizer Inc, whose projects all grabbed headlines by moving to human trials as early as March.
Sanofi has received financial backing from the United States and caused a stir in its home base of France after its British CEO Paul Hudson signalled that Europe was being too slow in supporting work on a vaccine, hinting U.S. patients might get any vaccine it develops first.
The EU is also seeking a deal with Sanofi, and has invited Britain to join its efforts to buy vaccines for the bloc. ($1 = 0.8011 pounds)