The Queen has ventured into public for the first time in the pandemic as London was placed under new restrictions, with the virus “spreading rapidly in every corner of our city.”

London mayor Sadiq Khan invoked a ban on socialising indoors which affects mixing at home and public places such as pubs and restaurants.

“I must warn Londoners that we’ve got a difficult winter ahead,” Mr Khan said.

“Nobody wants to see more restrictions but this is deemed to be necessary in order to protect Londoners’ lives by myself, London council leaders and by ministers.”

With the new measures, the capital has moved to the second-highest level in England’s three-tier warning system alongside other regions such as York and Essex, Health Minister Matt Hancock told parliament on Thursday.

“We must take firm and balanced decisions to keep this virus under control,” Mr Hancock said.

“Things will get worse before they get better but I know that there are brighter skies and calmer seas ahead,” the minister added.

Empty tables outside bars and restaurants in Covent Garden, in central London, after Mayor Sadiq Khan announced fresh measures to curb COVID. Photo: Getty

It comes as the World Health Organisation (WHO) urged European nations to “step up” and “don’t hold back” with restrictions as the second wave makes a resurgence on the continent.

As Mr Khan called for a nationwide lockdown to contain COVID, Queen Elizabeth’s visited a military research facility for her first major engagement since the coronavirus lockdown in March.

She made a rare joint appearance with her grandson Prince William, with neither wearing facemasks although all involved were physically distanced.

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

The Queen and grandson Prince William a rare public appearance together. Photo: Getty

The top secret Defence Science and Technology Laboratory, in Porton Down, southwest England, was involved in dealing with a 2018 Novichok nerve agent attack.

The facility 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.

Since March, the Queen’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.

On Thursday, following 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.

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.

-with AAP