Philip, 99, was admitted to King Edward VII’s Hospital in London on Tuesday evening as a precautionary measure after feeling unwell. The duke is said to have walked in unaided.
His eldest son, the Prince of Wales, was the first member of the royal family to visit when he made the 200-mile round-trip from his home in Highgrove, Gloucestershire, on Saturday afternoon.
It is understood that Charles wanted to visit his father due to Philip’s extended stay.
The prince arrived in a grey Tesla car at around 3.20pm on Saturday and got out of the vehicle wearing a face mask, before leaving the hospital around 30 minutes later.
A Clarence House spokesman confirmed he returned to Highgrove following his visit.
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 the Queen.
The visit came the day after Charles’s son and daughter-in-law, the Duke and Duchess of Sussex, were stripped of their prestigious patronages, as the couple confirmed Megxit was permanent.
The duke, who will turn 100 on June 10, is in hospital for an undisclosed reason, although it is not coronavirus-related.
The hospital’s website states visitors will “only be considered in exceptional circumstances”.
Regarding Charles’s visit, Buckingham Palace said it had nothing to add to its previous statement on the duke’s admission to hospital.
Philip is known for his “no fuss” attitude.
Royal author Penny Junor told the PA news agency: “We do all know he doesn’t like fuss, and he would regard a visit as fuss.”
She said she did not know the reason for the visit, but added: “Momentous things are happening in the family at the moment and I suppose it’s perfectly possible that Charles wanted to go and talk to his father and reassure him about Harry.”
Charles spoke last year of not being able to see his father as coronavirus restrictions persisted.
The prince, who spent much of the first lockdown at Birkhall in Scotland with the Duchess of Cornwall, said in June 2020: “Well, I haven’t seen my father for a long time.
“He’s going to be 99 next week, so, yes, or my grandchildren or anything. I’ve been doing the FaceTime, is all very well but…
“It isn’t the same, is it? You really want to give people a hug.”
At Christmas, Buckingham Palace confirmed that the Queen and Philip spent the holiday period together at Windsor, apart from the rest of the royal family.
In early December, the Queen had a socially-distanced royal reunion with the Duke and Duchess of Cambridge, Charles and Camilla, the Earl and Countess of Wessex, and the Princess Royal at an outdoor Christmas carol concert at Windsor at the end of William and Kate’s royal train tour.
But there was no sign of Philip at the festive gathering.
Ahead of Christmas, the duke released a rare public message praising teachers and school staff for their efforts teaching the nation’s children during the pandemic.
On Friday, sources told PA: “Following consultation with his doctor, the Duke of Edinburgh is likely to remain in hospital for observation and rest over the weekend and into next week.
“As we have said previously, the doctor is acting with an abundance of caution. The duke remains in good spirits.”
Earlier on Saturday afternoon, 10-year-old Twanna Saleh, from Camden, north London, wished the duke a “quick recovery” from outside the hospital with a home-made sign.