Prince Philip has spent a second night in hospital after being admitted earlier this week as a “precautionary measure” following complaints that he was feeling unwell.

Buckingham Palace did not issue an update on the Duke of Edinburgh’s health when contacted by CNN on Thursday.

The 99-year-old husband of Queen Elizabeth was admitted to King Edward VII Hospital in London on Tuesday evening, Buckingham Palace said in a press release, issued Wednesday.

According to a Royal source, it was not an emergency admission and the Duke traveled to hospital by car and walked in unaided. The source added that the illness was not Covid-19 related.

The Buckingham Palace press release on Wednesday said the Duke’s admission was a “precautionary measure” taken on the advice of his doctor after the Prince was “feeling unwell.”

“The Duke is expected to remain in hospital for a few days of observation and rest,” the press release added.

Prince Philip stepped back from public life in 2017 and, due to his advancing age, has been taken to hospital several times in recent years.

In December 2019, he received hospital treatment for a pre-existing condition.

The Duke was born in Corfu, Greece, in 1921 and has English, Russian, German/Prussian and Danish ancestry. He and Queen Elizabeth married in November 20, 1947.

Prince Philip and Queen Elizabeth have both received at least one dose of a Covid-19 vaccine.