TORONTO -- Prince Philip, the Duke of Edinburgh, was transferred to another hospital in central London on Monday where he will undergo testing and observation for a pre-existing heart condition, Buckingham Palace says.

The 99-year-old husband of Queen Elizabeth II has been receiving treatment for an unspecified infection at King Edward VII's Hospital in London since Feb. 17. According to The Associated Press, he is now under the eye of doctors at St. Bartholomew's Hospital, which specializes in cardiac care.

The palace says Philip "remains comfortable and is responding to treatment but is expected to remain in hospital until at least the end of the week."

The Duke’s illness is not believed to be related to COVID-19. Both he and the queen, 94, received a first dose of a coronavirus vaccine in early January.

Philip was initially admitted to the hospital less than two weeks ago as a precautionary measure after feeling unwell. In an interview with CTV News Channel, Montreal cardiologist Dr. Christopher Labos said that health complications can become more difficult with elderly patients.

“The older you are, the easier or the more likely it is that you’re going to have complications and the longer it’ll take you to recover,” he explained. “The problem with any infection is that if it’s severe enough it can put strain on the heart.”

Labos said that for people with pre-existing conditions, the added bodily stress of an infection can increase the demand for oxygen and make it harder for the heart to function properly.

“Even in the absence of having an actual heart attack, just the infection itself can put strain on the heart and complicate the situation,” he added.

The Barts Heart Centre at St. Bartholomew's Hospital is Europe's biggest specialized cardiovascular centre, according to the National Health Service’s website. The centre seeks to perform more heart surgery, MRI and CT scans than any other service in the world.

Philip, who married the then-Princess Elizabeth in 1947 and is the longest-serving royal consort in history. In 2017, he retired from royal duties and is rarely seen in public.

During the U.K.’s current COVID-19 lockdown, he has been staying with the Queen at Windsor Castle, west of London.

The Duke’s health issues date back to 2011, when he was rushed to a hospital by helicopter after experiencing chest pains and was treated for a blocked coronary artery. In 2017, he spent two nights in the hospital, and he was hospitalized for 10 days in 2018 for a hip replacement.

Philip was last hospitalized in December 2019, spending four nights at King Edward VII's Hospital for what the palace said was planned treatment of a pre-existing condition.

With files from The Associated Press.