The 95-year-old monarch had been due to spend two days in Northern Ireland this week but was forced to cancel the visit after being advised to rest for a few days.
It is understood she was seen by specialists on Wednesday at the private King Edward VII’s Hospital in central London for some preliminary investigations, and that the overnight stay was for practical reasons.
Informed sources said the Queen was back at her desk yesterday afternoon and was undertaking light duties.
There was no sense of alarm about her condition, and she fully intends to host world leaders at the Cop26 climate summit in Glasgow, alongside the Prince of Wales and Prince William.
Her hospitalisation is not believed to have been related to Covid-19. Buckingham Palace was last night effectively bounced into revealing the Queen had stayed overnight in hospital after claiming she was at home resting.
Her aides decided to issue a “media advisory” statement just after 10pm after the Sun newspaper learned of her stay.
It stated: “Following medical advice to rest for a few days, the Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today, and remains in good spirits.”
It is understood that the Palace had not planned to release the fact of the overnight hospital stay to the press, despite previous hospital visits by her and Prince Philip being made public in the past.
Just days earlier the Queen was seen using a walking stick in public at a Westminster Abbey service to honour the Royal British Legion.
At the time the Palace said that there was no “particular” reason for the stick other than the Queen using it for “comfort”. She used the cane again when visiting the Welsh Parliament a few days later.
The last time the Queen was pictured using a stick was in 2003, after surgery to remove a torn cartilage from her right knee.
It has also been reported that the Queen has given up her evening alcoholic drink, usually a martini, on the advice of her medical team.
Even though individuals have a right to medical privacy, it is usual for the public to be informed if senior public figures are hospitalised.
When Prince Charles contracted Covid at the beginning of the pandemic in March 2020, his advisers decided to release the information.