Prince Philip has been admitted to the King Edward VII Hospital in London Tuesday night as a precautionary measure after "days of feeling unwell" but Buckingham Palace said his condition is not related to COVID-19.

Palace sources confirmed that the Duke of Edinburgh, who is 99 years old, was driven to the hospital upon the advice of royal doctors. However, Philip remained "in good spirits" as he got into the hospital unassisted.

The Queen's husband is expected to stay at the hospital for a few days to rest and be observed. Her Majesty has remained in isolation at Windsor Castle and continued to carry out her work. Queen Elizabeth was, reportedly, speaking on the phone on work-related matters with a Royal Navy admiral in the morning.

The royal couple received their first COVID-19 vaccinations in early January 2021, prompting Prince William to say that he is proud of his grandparents for getting inoculated. It is believed that the Queena and her husband recently had their second jab but the Palace won't reveal what vaccine they received.

Prime Minister Boris Johnson led the United Kingdom in wishing the Queen's husband well while guards have been spotted outside the private hospital where Philip is receiving treatment. The last time the Duke of Edinburgh was in the hospital was days before Christmas 2019, where royal doctors also confirmed that he had to be admitted as a precautionary measure for a pre-existing condition.

Since the pandemic, the Queen and her husband had been staying in isolation at Windsor Castle. The royal couple briefly stayed under a protective bubble at Philip's house in Wood Farm in Sandringham in October 2020.

Meanwhile, royal courtiers at the Palace are preparing for Philip's milestone 100th birthday in June but he doesn't want any fuss about it and has been tagged as a "reluctant celebrant." Insiders said that the Duke of Edinburgh wants "nothing to do" with the planned celebration thus they have been challenged to plan out the event.

But sources said this is not surprising behavior from Philip, who never got comfortable with the fuss that goes with his status as the Queen's consort. In a 2000 interview, Philip also said that turning 100 might be the worse as bits of himself are "falling off" as it is.

In the last decade, the Duke of Edinburgh has had major hospitalizations for a blocked coronary artery, a bladder infection and surgery in his abdomen. Philip has also been wearing a hearing aid since 2014. He retired from his public duties in 2017 and has stayed away from royal events since except for one event last summer when he turned over a royal role to his daughter-in-law, Camilla, the Duchess of Cornwall.