The 94-year-old monarch, who spent Britain’s national lockdown at her Windsor Castle home, has carried out her official duties remotely by video or telephone, or at a royal residence, since social restrictions were introduced.

For her first public outing, she made a rare joint appearance with her grandson Prince William, with neither wearing facemasks although all involved were socially distanced and staff had been tested for COVID-19 prior to their visit.

The decision not to wear a mask drew criticism from some as Britons are required to wear face coverings in shops and other indoor venues, while others said it gave a positive message to the public when more restrictions are being imposed.

“She is sending a message of confidence to the people; she’s going back to work, she’ll go about her business in the usual way, but without taking chances,” royal author Robert Jobson told the Daily Mail website.

The top secret Defence Science and Technology Laboratory, in Porton Down, southwest England, identified the nerve agent used to attack Russian ex-spy Sergei Skripal and his daughter Yulia in the nearby city of Salisbury two years ago.

Britain has accused two Russian agents backed by Moscow of carrying out the attack, a charge the Kremlin has rejected. Russian officials are also accused of using Novichok to poison Kremlin critic Alexei Navalny in August.

During their trip, the royals met the Porton Down staff and military personnel who were involved in the Novichok clear-up operation, along with scientists helping the response to the COVID-19 pandemic.

The formal purpose of the trip was to open the lab’s new Energetics Analysis Centre, where the queen and the prince were given a demonstration of a forensic explosives investigation.

They also toured the lab’s Energetic Enclosure to see a display of weaponry and tactics used in counter intelligence.

Porton Down was founded in 1916 as Britain’s chemical and biological weapons laboratory, where defence ministry scientists carried out secret experiments involving some 20,000 servicemen, leading to the death of one from exposure to sarin nerve gas.

Since the 1960s, it has focused on developing countermeasures and defence and security technology, although its secret structure has always led to speculation about its activities, especially among flying-saucer theorists.

“No aliens, either alive or dead have ever been taken to Porton Down,” the government’s website says.

Since March, Elizabeth’s only public appearances have been at Windsor for a military ceremony in June to mark her birthday, and for a ceremony to knight record-breaking charity fundraiser, 100-year-old Captain Tom Moore.

She has also paid private visits to Balmoral Castle in Scotland and Sandringham in eastern England, her private residences.

On Thursday, in keeping with the routine of much of her 68-year reign, she unveiled a plaque at Porton Down and signed the guest book.

“Well, it proves we’ve been here, doesn’t it?” she joked.