The trial, set to begin this week, will compare Valneva’s vaccine against the already-approved jag made by AstraZeneca.
It comes after positive results from Valneva’s phase 1 and 2 trials, with the vaccine well-tolerated and no safety concerns identified.
The UK has ordered 100 million doses of the Valneva vaccine, which are set to be delivered at the end of 2021 and beginning of 2022.
With all adults due to have been vaccinated by the end of the summer, Valneva hopes its vaccine will be used as a booster jag or as a modified vaccine which is more effective against new variants of Covid-19.
The phase 3 trial will run in 24 sites across the UK, with two in Scotland. It is open to healthy adults who have not already had a vaccine.
Around 3,000 people over 30 will be given either two doses of the Valneva vaccine or two doses of the AstraZeneca jag.
Following JCVI guidance not to offer AstraZeneca to under 30s, around 1,000 younger participants in the study will be given only the Valneva option.
Volunteers will be given two doses, 28 days apart, starting at the beginning of May.
Thomas Lingelbach, Valneva chief executive, said: “This Phase 3 initiation marks a significant milestone in the development of the only inactivated vaccine candidate against Covid-19 in clinical trials in Europe.
"As Covid-19 continues to impact people’s daily lives, we remain fully focused on developing another safe and efficacious vaccine solution.
"We believe that VLA2001 has an important role to play including boosters or potential modifications to the vaccine to address variants.”
Valneva’s candidate is an inactivated whole virus vaccine, which contains virus that has been destroyed so cannot infect cells, but can still trigger an immune response.
The technology is used in flu, polio and rabies vaccines, and it’s a more traditional approach than the Pfizer (mRNA) and AstraZeneca (adenoviral) vaccines.
Because the vaccine doesn’t contain any live virus, it may be especially suitable for vulnerable people, such as the elderly or those with weaker immune systems.