The Queen spent a night in hospital after being advised by her doctors to cancel official duties.
Her Majesty was due to visit Northern Ireland but was told to rest.
Doctors said the 95-year-old monarch should “take it easy” for a few days.
The Queen was taken by car to the King Edward VII Hospital in Marylebone, central London
A Buckingham Palace spokesman said:
“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’s reported the overnight stay was for practical reasons rather than a medical concern.
The Queen is still expected to attend the Cop26 climate talks in Glasgow at the end of October.