Sharjah24 – Reuters: Britain's 95-year-old Queen Elizabeth spent a night in hospital for the first time in years for what royal officials termed 'preliminary investigations' but was in good spirits and back at work at her Windsor Castle home on Thursday.

The world's oldest and longest-reigning monarch cancelled an official trip to Northern Ireland on Wednesday. The palace said the queen had been told to rest by her medical staff, and that her ailment was not related to COVID-19.

"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 palace said in a statement on Thursday.

A royal source said the queen had stayed at the King Edward VII hospital in central London for practical reasons and that her medical team had taken a cautious approach.

Elizabeth, who is queen of 15 other realms including Australia, Canada and New Zealand, returned to her desk for work on Thursday afternoon and was undertaking some light duties, the source said.

She had spent Tuesday night hosting a drinks reception at Windsor for billionaire business leaders such as Bill Gates after Prime Minister Boris Johnson convened a green investment conference ahead of the COP26 climate summit.

The queen had appeared in good health then, smiling happily as she met the guests.

The head of state, who next year celebrates 70 years on the throne, is known for her robust health. The last time she is thought to have spent a night in hospital was in 2013 when she was suffering from symptoms of gastroenteritis.

She had a successful surgery to treat an eye cataract in 2018, and also had a knee operation in 2003. However, royal officials are loathe to discuss health issues in general, saying medical matters are private.

Earlier this month, she was seen using a walking stick for support in public for the first time, apart from after her knee operation.