The Queen, 95, spent Wednesday night in hospital for "preliminary investigations". But the monarch is said to have already been back to work after returning to Windsor Castle at lunchtime on Thursday.

       A source told the Daily Mail: "Quite understandably, a cautious approach is being taken by the medical team.

       'Her Majesty was back at her desk this (Thursday) afternoon and undertaking light duties."

       The Queen's hospital visit came after she cancelled a trip to Northern Ireland following doctors' orders to rest.

       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."

       The head of state's trip to central London's private King Edward VII's Hospital did not emerge until Thursday evening when it was reported in The Sun, prompting the palace to issue a statement.

       It was the monarch's first overnight hospital stay in eight years.

       She spent a night at the private clinic in 2013 when she was treated for a bout of gastroenteritis.

       The Queen travelled to the hospital this week by car rather than by helicopter.

       READ MORE: Queen had no choice but to rest says expert

       It is understood the head of state was due to attend for only a short stay while she was seen by specialists, so the development was not announced by the palace at the time, and protecting the Queen's medical privacy was also a consideration.

       A source said the overnight stay was for "practical reasons".

       King Edward VII's Hospital is 23 miles from Windsor - around a 55-minute drive in good traffic conditions.

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       The Queen's team of royal physicians are understood to have been taking a "cautious approach".

       Her admission is understood not to have been related to coronavirus.

       The Queen has had a packed royal schedule in recent weeks.

       She hosted a major Global Investment summit at Windsor Castle on Tuesday evening.

       Last week, the Queen used a walking stick for what is believed to be the first time at a major public event.

       The monarch, who is just five years away from her 100th birthday, is due to celebrate her Platinum Jubilee - 70 years on the throne - next year.

       She still keeps a busy diary of events and audiences and deals with her daily red boxes of official papers.