Buckingham Palace said in a statement that doctors "will continue to treat him for an infection, as well as undertake testing and observation for a pre-existing heart condition."
"The Duke remains comfortable and is responding to treatment but is expected to remain in hospital until at least the end of the week," the statement said.
Philip, who's official title is the Duke of Edinburgh, is 99 years old. He had been receiving treatment at the private King Edward VII's Hospital. He will now be at St Bartholomew's Hospital which is run by the National Health Service. According to its website, Barts Heart Centre is Europe’s largest specialized cardiovascular service.
The last statement the palace released was on Tuesday when it said that the duke was “comfortable and responding to treatment.” It did not name the type of infection he had or say anything about its severity, though it did say that he was expected to stay for several more days.
The day the statement was released, Philip’s son Prince Edward said his father was “a lot better.”
“He's looking forward to getting out which is the most positive thing. So we'll keep our fingers crossed,” Edward told Sky News.
Prince Charles, the royal couple’s first born and heir to the throne, visited his father on Feb. 20.
The queen has carried on with her duties during Philip’s time in hospital. Last week, she encouraged the public to get the COVID-19 vaccine in a video call with medical officials.
"Once you've had a vaccine you have a feeling of you know, you're protected which I think is very important and as far as I could make out it was quite harmless," the queen said.
"It was very quick, and I’ve had lots of letters from people who have been very surprised by how easy it was to get the vaccine. And the jab — it didn’t hurt at all," she added, likening the virus to a plague.
The queen and Philip both received their Covid-19 vaccinations in January.
During the coronavirus pandemic, the royal couple have been staying at Windsor Castle, around 30 miles west of London. The U.K. has the highest death toll in Europe.
Philip retired from public duties in 2017, and is the longest-serving royal consort in British history.
In April, Philip released a written statement praising those involved in the fight against the coronavirus.
Philip and the queen celebrated their 73rd anniversary in November. Together they have four children — Charles, born in 1948; Princess Anne, born in 1950; Prince Andrew, born in 1960; and Edward, born in 1964 — along with dozens of grand and great-grandchildren.
Philip has had a number of health issues in recent years and was last hospitalized in December 2019, when he spent four nights in the hospital for what the palace said was planned treatment of a pre-existing condition, although it did not reveal what that condition was.
This article was originally published on NBCNews.com.