French-Austrian biotech firm Valneva said Wednesday it will begin clinical trials of its Covid-19 vaccine candidate in Britain, where the government has already ordered at least 60 million doses.

The specialist vaccine developer said the phase 1 and 2 trials, involving around 150 healthy adults, would focus on the drug's safety and ability to stimulate an immune response at three different doses.

Supported by Britain's National Institute of Health, the tests will takes place at several sites around the UK.

Valneva hopes to determine the optimal dose of the vaccine in the second quarter of next year, putting it on track for approval by the autumn -- several months later than its previous mid-2021 ambition.

As well as a firm order for 60 million doses, Britain has options to buy a further 130 million between 2022 and 2025 in a deal worth up to 1.4 billion euros ($1.7 billion, £1.2 billion).

Valneva's vaccine contains a deactivated form of the virus that can prompt an immune response from the body but cannot harm health, the same technique used in its existing inoculation against Japanese encephalitis.

Based in Nantes in western France, Valneva employs around 500 people worldwide and already has two vaccines on the market, for Japanese encephalitis and cholera.

It has a further vaccine against Lyme disease in phase 2 trials and a fourth, for the chikungunya virus, in phase 3.

Valneva is already listed on the Paris stock market and aims to secure a New York listing next year.