The 94-year-old monarch was accompanied by her grandson Prince William on a visit to the Defence Science and Technology Laboratory (Dstl) at Porton Down in southwest England, which has also been conducting research into coronavirus.

They met staff involved in identifying Novichok as the nerve agent used to try to kill Russian double agent Sergei Skripal in the nearby town of Salisbury in 2018, and members of the military who helped in the incident.

Skripal and his daughter Yulia survived the attack but a homeless woman who came into contact with a perfume bottle thought to have contained the nerve agent died four months later.

Britain and its allies blamed Russia and the incident prompted a mass expulsion of Russian diplomats by Western nations. Moscow denied any involvement.

The queen and William, neither of them wearing face masks but walking two metres apart, also met scientists involved in analytic research in support of Britain's response to the coronavirus outbreak.

It was the monarch's first public engagement outside one of her homes since she attended a Commonwealth Day service at Westminster Abbey on March 9.

As coronavirus swept across the UK, she and her husband Prince Philip, 99, retired first to Windsor Castle west of London, as a precaution because of their age.

They then spent time at her residences at Balmoral, northeast Scotland, and Sandringham, in eastern England, before returning to Windsor last week.

Aides said all 48 people due to come into close contact with the monarch and William on Thursday were tested for Covid-19 before the event.

The queen made two televised addresses to the nation early on in the outbreak, which has killed more than 43,000 people in Britain -- the highest toll in Europe.

At Porton Down, she formally opened the new Energetics Analysis Centre, an explosive materials research centre.

She and William were due to be shown displays of weaponry and tactics used in counter intelligence.