The monarch was celebrating her 95th birthday in muted fashion following the death of her “beloved” husband the Duke of Edinburgh, who was laid to rest on Saturday.
It is the first time in her reign that her “strength and stay” Prince Philip has not been by her side.
Members of the royal family have been supporting the Queen, and some were expected to visit her at Windsor.
The birthday falls within the two-week period of royal mourning, which is being observed until Friday.
Watch: Queen’s 95th birthday to be private and low-key
Prince Charles and his sister Anne, the Princess Royal, who is grieving at her Gatcombe Park estate in Gloucestershire, are expected to telephone their mother.
Following the funeral, Charles, 72, travelled to his Welsh retreat at Llwynywermod in Llandovery without his wife Camilla who had private commitments in London.
Prince Harry, who flew in from the US for his grandfather’s funeral, is believed to have returned home yesterday to be with pregnant wife Meghan and their son Archie.
Harry, 36, is reported to have flown into Los Angeles on an American Airlines flight, and his car was seen arriving at his home in Montecito, California, a short time later.
Princes William and Harry took “baby steps” towards healing their relationship at the weekend after they walked together back to Windsor Castle from the funeral at St George’s Chapel.
Prince Andrew, who is very close to the Queen and lives near Windsor, is expected to see his mother today.
Prince Edward and his wife Sophie, Countess of Wessex are also expected to visit her while complying with Covid-19 guidelines.
The Queen will work from Windsor Castle over the coming weeks. She is expected to be focused on responding personally to messages from foreign heads of state and leading political figures who expressed their condolences for the duke’s death.
Today is the Queen’s real birthday. The pandemic has meant that her official birthday celebration, Trooping the Colour, normally staged in June, has been cancelled for a second year running.
Buckingham Palace released a photo of the Queen to mark her birthday on the official royal family Instagram account today. It shows the Queen smiling in a burgundy ensemble at a royal engagement.
On Twitter today Boris Johnson wrote: “I would like to send my warm wishes to Her Majesty The Queen on her 95th birthday.
“I have always had the highest admiration for Her Majesty and her service to this country and the Commonwealth.
“I am proud to serve as her Prime Minister.”
Watch: Who is The Queen?