Queen Elizabeth II carried out her first public engagement since Britain's coronavirus lockdown in March, stepping out on Thursday without a mask despite a resurgence of the virus.

The 94-year-old monarch was joined by her grandson Prince William on a visit to the Defence Science and Technology Laboratory at Porton Down, Southwest England. Neither were wearing face coverings, though they were adhering to social distancing guidelines.

The UK government recommends wearing a face covering in indoor places where "social distancing may be difficult and where you will come into contact with people you do not normally meet."

Buckingham Palace said that the Queen decided not to wear a mask after consulting her own medics and scientists at the Porton Down military research facility, which is providing vital support to the United Kingdom's Covid-19 pandemic response.

"Specific advice on this visit has been sought from the medical household and relevant parties," a royal source added.The monarch, who spent the national lockdown at Windsor Castle with her husband, Prince Philip, has been carrying out her official duties online -- including attending a virtual portrait unveiling, and holding a video call with members of the armed forces.

The royal family was directly impacted by the pandemic in March, when Prince Charles, Queen Elizabeth II's eldest son and heir to the British throne, tested positive for coronavirus. Charles, 71, completed his self-isolation in Scotland.

Prince Charles' office said at the time that it was unknown how he caught the virus because of his busy schedule of public events. Britain is currently grappling with a spike in Covid-19 cases.

UK Prime Minister Boris Johnson on Wednesday imposed a three-tier Covid Alert system across England to try to staunch the spread of the virus, rolling out localized restrictions in some northern cities and the capital London.

In England, face coverings are compulsory in most indoor settings, including visitor attractions and entertainment venues, and in pubs and restaurants, unless seated to eat and drink.

According to the government, people in England are expected to wear a face covering before entering such settings, "unless there is a reasonable excuse for removing it."