The 95-year-old monarch, wearing a bright pink floral outfit, greeted the Bidens in the Quadrangle of the castle, home to the royal family for nearly 1000 years and where she has spent much of her time since the outbreak of the coronavirus pandemic.

Giving the US president and first lady a taste of British pomp on their arrival, a Guard of Honour formed of The Queen's Company First Battalion Grenadier Guards gave a Royal Salute, and the US national anthem was played.

The president and the Queen looked relaxed and chatted before they headed inside the castle for Sunday's private meeting.

Queen Elizabeth had already met the Bidens on their visit to Britain.

She hosted a reception for leaders of the Group of Seven rich nations and their spouses on Friday night in the southwestern English county of Cornwall where the three-day summit was held.

"Joe and I are both looking forward to meeting the Queen," Jill Biden said after they arrived in Britain on Thursday. "That's an exciting part of the visit for us."

Joe Biden and his wife will become the fourth president and first lady the Queen has received at Windsor, following the Trumps in 2018, the Obamas in 2016, President George W Bush and his wife in 2008 and the Reagans in 1982.

During her 69 years on the British throne, the longest reign of any English monarch, there have been 14 US presidents, of which Elizabeth has met all bar Lyndon Johnson.

Despite the loss in April of Prince Philip, her 99-year-old husband of more than 70 years, and her age, the Queen has shown her determination to carry on with her official duties as head of state during the high-profile G7 gathering.

After hosting the leaders on Friday where she cracked a joke during the official photo call, she provoked more laughter at another official engagement for a local project immediately afterwards when she cut a cake with a long ceremonial sword.

On Saturday, she was back in Windsor for a military parade to mark her official birthday, the first one she has marked without Philip.

After leaving Windsor, Biden will head off to a NATO summit in Brussels on Monday, a US-EU summit on Tuesday and a meeting with Russian President Vladimir Putin in Geneva the following day.

Australian Associated Press