Prince Charles Visits His Father Prince Philip in the Hospital on Fourth Night of Recovery

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Prince Charles visited his father Prince Philip at the London hospital where the 99-year-old royal has been staying since being admitted earlier this week.

On Saturday, Charles, 72, was photographed arriving at the King Edward VII Hospital wearing a gray suit and a face mask to stay safe amid the coronavirus pandemic. The heir to the throne was seen entering the rear entrance of the facility alongside security personnel.

Queen Elizabeth's husband was taken to the hospital on Tuesday evening on the advice of his doctor after "feeling unwell," Buckingham Palace said in a statement at the time. The royal was expected to remain in the hospital for a few days.

On Friday, a royal source told PEOPLE that Philip is now expected to remain in the hospital "into next week."

RELATED: Prince Philip to Remain in Hospital 'Into Next Week': He's 'in Good Spirits,' Says Royal Source

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"Following consultation with his doctor, the Duke of Edinburgh is likely to remain in hospital for observation and rest over the weekend and into next week," the source said. "The doctor is acting with an abundance of caution. The Duke remains in good spirits."

Palace sources stressed after Tuesday's admission to the hospital that this was not an emergency admission and that Philip walked into the hospital unaided. The illness is not COVID-19-related.

Both the Queen, 94, and Philip received the first dose of the coronavirus vaccine in early January.

Philip, who will turn 100 in June, has been plagued by health issues in recent years. According to The Sun, Philip has been treated at King Edward VII Hospital several times in the past.

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He was hospitalized in December 2019 in relation to a "pre-existing condition," according to the palace at the time.

In March 2018, Philip abruptly canceled his appearance at the traditional Maundy church service that he was scheduled to attend with the monarch. Philip also skipped Easter Sunday services that year ahead of the announcement that he underwent hip replacement surgery.

RELATED VIDEO: Prince Philip, 99, Admitted to Hospital in London After 'Feeling Unwell'

Prince Philip, 99, Admitted to Hospital in London After 'Feeling Unwell'

"The Duke's admission is a precautionary measure, on the advice of His Royal Highness's Doctor, after feeling unwell," Buckingham Palace says in a statement

In January 2019, Philip had a car accident that resulted in him voluntarily giving up his license.

Amid Philip's hospitalization, the Queen has remained at Windsor Castle, where the couple has spent much of the year in isolation during the coronavirus pandemic.

In early November, the couple returned to Windsor Castle together after spending a weekend at Sandringham. They had spent a few weeks apart in October when Philip stayed at Wood Farm on her estate in Norfolk while the Queen went to Windsor Castle.