"Today is the queen’s 95th birthday," the palace tweeted on Wednesday.
"[Her majesty] was born on 21 April 1926 at 17 Bruton Street in London, the first child of the duke and duchess of York.
"This year the queen remains at Windsor Castle during a period of royal mourning following the death of the duke of Edinburgh," it added.
The queen will not have a public celebration.
The reigning monarch is understood to have "no plans" to mark the event on Wednesday, which is her first birthday without her husband, Prince Philip, in 73 years.
Philip, who was also known as the duke of Edinburgh, was laid to rest on Saturday in a military-style funeral after he died on 9 April, aged 99.
After his death was announced, Britain entered a week-long period of mourning, which ended after his funeral on Saturday.
The Royal Standard flies over Windsor Castle, on Queen Elizabeth II's 95th birthday. Photo: Steve Parsons/dpa.
The royal family has a two-week period of mourning, which ends on Friday.
Prince Harry, the queen and Prince Philip's grandson, flew in from Los Angeles to attend the funeral.
He was reunited with his family for the first time since his bombshell interview with Oprah Winfrey, in which he revealed that he stepped down as a senior royal because of negative and racist press coverage they received.
A spokesperson for Prince Harry declined to say if the prince remained in England to celebrate her birthday, however, British newspaper The Telegraph said he was expected to fly back on Tuesday or Wednesday.
Last year, a gun salute to mark the queen's 94th birthday, which usually takes place Hyde Park and the Tower of London in the capital, was cancelled as the queen felt it was not appropriate during the pandemic.
It is not clear if the annual celebration for her official birthday, known as Trooping the Colour, will take place in June.