Britain's Queen Elizabeth spent Wednesday night in hospital for "preliminary investigations", Buckingham Palace says.
The 95-year-old monarch, who was ordered to rest by doctors and advised to miss a trip to Northern Ireland, returned to Windsor Castle at lunchtime on Thursday, and remains in "good spirits".
She was seen by specialists at the private King Edward VII's Hospital in central London, Her ailment was not related to COVID-19.
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 is understood the trip to hospital on Wednesday afternoon was expected to be for a short stay for some preliminary investigations, so was not announced by the palace at the time, as well as in order to protect the Queen's medical privacy.
The overnight stay was said to be for practical reasons.
The Queen's medical team is understood to have been taking a cautious approach.
She was back at her desk this afternoon, undertaking light duties.
The monarch was said to be disappointed not to be able to travel to Northern Ireland on Wednesday, and reluctantly heeded the advice of her royal physicians.
She has had a busy schedule since returning from Balmoral at the start of October, and hosted a major Global Investment summit at Windsor Castle on Tuesday evening, where she looked bright and cheerful as she carried out her royal duties.
The head of state, who next year celebrates 70 years on the throne, is known for her robust health and the last time she is thought to have spent a night in hospital was in 2013 when she was suffering from symptoms of gastroenteritis.
Earlier this year, Prince Philip, her 99-year-old husband of more than seven decades, died at Windsor Castle.
With Reuters.
