LONDON — Britain's Queen Elizabeth II returned to her home at Windsor Castle home on Thursday after spending a night in hospital, Buckingham Palace said.

The 95-year-old monarch spent a night in hospital for the first time in years for what Buckingham Palace termed "preliminary investigations".

The world's oldest and longest-reigning monarch canceled 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 late on Thursday.

A palace 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.

The Palace gave no details on what had prompted the medical attention, but it was not related to COVID-19.

The queen has been vaccinated against the coronavirus.

The palace confirmed the queen's hospital stay after The Sun newspaper reported the news.

The queen is now back at Windsor Castle, west of London, where she has spent much of her time since the onset of the COVID-19 pandemic last year.

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 acceded to the throne as Britain was shedding its imperial power, and has symbolized stability for generations of British people. — Agencies