Video report by ITV News Royal Editor Chris Ship
Prince Philip was admitted to King Edward VII's on Tuesday evening as a precautionary measure and walked unaided into the medical centre.
The Prince of Wales arrived at the hospital at around 3.20pm on Saturday in a grey Tesla car, and exited the vehicle wearing a face mask.
He is the first member of the royal family to visit Philip during the Duke's four-night stay in hospital.
It is understood the Prince travelled from his home in Highgrove, Gloucestershire, to the hospital in central London, a journey of around 100 miles.
A spokesperson for Prince Charles confirmed he returned to Highgrove after the visit. It is understood the Prince of Wales wanted to pay a visit to see his father in light of Philip's extended stay at the hospital.
It is thought Charles had not seen his father since before Christmas because of the nationwide coronavirus restrictions, with the Duke staying at Windsor Castle.
With tough coronavirus restrictions and visiting rules in place. ITV News Royal Editor Chris Ship says Prince Charles's visit has raised several unanswered questions
Charles left the hospital shortly before 4pm on Saturday afternoon, around 30 minutes after arriving.
The hospital's website states that visitors will "only be considered in exceptional circumstances".
There has been no update from Buckingham Palace on Prince Philip's condition and no information on what the 99-year-old is being treated for, but royal sources have been keen to emphasise it wasn't an emergency.
It has been confirmed Prince Philip - who received his coronavirus vaccination in early January - was not admitted for a Covid-related illness.
On Friday, royal sources told ITV News they expected Prince Philip to stay in hospital until next week.
According to the source the Duke is in "good spirits" and the measures were being taken as a precaution by his doctors.
The trip is his fourth visit to hospital since he retired in 2017.
Since his retirement, Prince Philip has spent most of his time out of the public eye and has suffered from a number of ailments.
He previously spent four nights at the same hospital he is in now in December 2019 for treatment on a “pre-existing condition.”
The morning after the Duke was admitted, the Prince of Wales and Duchess of Cornwall – who have received their Covid jabs – visited the Queen Elizabeth Hospital in Birmingham to learn about vaccination trials.
While on Thursday, the Queen carried out her first official in-person event of the year and made one of her most senior royal aides a Knight Commander of the Royal Victorian Order during a Windsor Castle ceremony.