LONDON, Oct 21 (Reuters) - Britain"s 95-year-old Queen Elizabeth spent a night in hospital for the first time in years for what Buckingham Palace termed "preliminary investigations" but returned to Windsor Castle on Thursday where she was in good spirits. 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. A royal source said the queen had stayed in hospital for practical reasons and that her medical team had taken a cautious approach. She returned to her desk for work on Thursday afternoon and was undertaking some light duties, the source said. Elizabeth 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 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. 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 year, Prince Philip, her 99-year-old husband of more than seven decades, died at Windsor Castle. But that has not stopped her from carrying out her official engagements, although her age has meant she has handed more duties to her son and heir Prince Charles and other members of the royal family. Earlier this month, she was seen using a walking stick for support in public for the first time, apart from after her knee operation, when she arrived at a service at London"s Westminster Abbey. Elizabeth, who acceded to the throne in 1952 as Britain was shedding its imperial power, has symbolised stability for generations of British people, maintaining the popularity of the monarchy despite seismic political, social and cultural change which threatened to make it an anachronism. A quiet and uncomplaining dedication to the duty of queenship, even in old age, has earned her widespread respect both in Britain and abroad, even from republicans who are eager to abolish the institution.