Thirteen days after he was first hospitalized, Prince Philip was transferred on Monday to a second London hospital, increasing concerns over the health of the 99-year-old husband of Queen Elizabeth.

Buckingham Palace said in a statement that doctors would continue to treat him for an infection but would also “undertake testing and observation for a pre-existing heart condition.”

Prince Philip “remains comfortable and is responding to treatment but is expected to remain in hospital until at least the end of the week,” the brief statement said, adding that he had been transferred to St. Bartholomew’s hospital in London.

He was originally taken from Windsor Castle, where he had been staying during the latest coronavirus lockdown, to King Edward VII’s Hospital on Feb. 16 after feeling unwell. He traveled to London by car and walked into the hospital unaided, rather than as an emergency admission for a visit that was described as a precaution. At the time Buckingham Palace said that he was expected to remain there for a few days of observation and rest.

Last week, the palace confirmed reports that Prince Philip was being treated for an infection while making it clear that this was not related to Covid-19. Prince Philip was vaccinated against Covid-19 in January, as was Queen Elizabeth, 94, who has urged others to take up offers of the vaccine.

The announcement Monday ends hopes that Prince Philip’s hospitalization would be limited to a brief stay, inevitably increasing anxiety about the condition of a patient who turns 100 in June.

According to British media reports, he was transferred by ambulance at around 11 a.m. on Monday to St. Bartholomew’s, an internationally renowned hospital with special expertise in cardiac care.

In December 2011, Prince Philip received treatment for a blocked coronary artery. Buckingham Palace said in a statement then that he had successfully undergone a “minimally invasive procedure of coronary stenting” after suffering chest pains.

In the last decade he has been admitted to hospital several other times, including for an infection in 2012, for abdominal surgery in 2013 and a hip replacement in 2018.

During his recent hospital stay Prince Philip has been visited by his son Prince Charles, who is heir to the throne, while other members of the royal family have made reassuring comments about the situation.

His youngest son, Prince Edward, said in an interview last week with Sky News that his father was “a lot better,” and was “looking forward to getting out which is the most positive thing, so we keep our fingers crossed.”

Asked whether Prince Philip, who is renowned for his blunt manner, was getting frustrated with life in hospital, Prince Edward replied with understatement and a laugh: “just a bit.”

The illness comes at a difficult time for the British monarchy following the announcement from Prince Harry and his wife, Meghan, that their departure from royal duties is to be permanent.

The couples are expected to give their reasons for the split in a prime-time interview with Oprah Winfrey that is scheduled to run in the United States on Sunday. In a brief clip of the conversation, released by CBS, Prince Harry drew parallels between the media’s treatment of Meghan and of his mother, Princess Diana, saying he had feared history was “repeating itself” before his decision to step back as a senior royal.