The Duke of Edinburgh is being looked after "very well" in hospital, the Prince of Wales has said.
The Duke of Edinburgh, 98, remains at the King Edward VII in central London after being admitted on Friday - he was not taken by ambulance.
He travelled from Sandringham, where he spends much of his time since retiring, as a "precautionary measure" following his doctor's advice.
During a visit to flood-hit communities in South Yorkshire on Monday, Prince Charles was asked about his father's health by a reporter.
"He's being looked after very well in hospital," Charles replied.
"At the moment that's all we know."
During the visit, a woman in Fishlake asked Charles: "Sir, how is your father?"
He replied: "Alright. When you get to that age things don't work so well."
On Saturday morning uniformed police officers stood guard outside the hospital.
Sky's Royal correspondent Rhiannon Mills said after his admission: "The King Edward VII is the hospital that is always used by the Royal Family.
"The palace will not give details about the condition he's being treated for, but the decision to send him to hospital in London may be related to where his consultants are based.
"I understand that at the moment he is likely to stay in hospital for a few days.
"It is worth noting that the Queen has not changed her plans. She left Buckingham Palace on Friday and travelled by train to Kings Lynn in Norfolk to begin her Christmas in Sandringham.
"She would have been travelling north with the knowledge that her husband was heading to London for treatment."
On Sunday, Her Majesty was accompanied by Sophie, Countess of Wessex as she attended a church service at St Mary Magdalene Church in Sandringham.
The Queen has been married to Prince Philip for more than 70 years.
He is known for his "no fuss" approach and is unlikely to receive many visitors while he is being treated at the hospital.
The prince has suffered ill health in recent years:
2008: Philip was admitted to hospital for a chest infection and was discharged three days later.
2011: Two days before Christmas, the duke was taken to hospital from Sandringham with chest pains. He had surgery and was discharged four days later.
2012: During the Queen's Diamond Jubilee celebrations Philip was taken to hospital with a bladder infection and discharged five days later. Two months later the infection recurred and he was taken to hospital as a precautionary measure.
2013: Philip had an exploratory operation on his abdomen, spending 11 days in hospital.
2014: Philip appeared in public with a bandaged right hand after a "minor procedure".
2017: The Prince was taken to hospital with an infection.
2018: The Duke of Edinburgh was admitted to hospital for a planned operation on his hip.
In January this year, the duke was involved in a car crash when the Land Rover Freelander he was driving flipped over after colliding with another car near the Sandringham estate.
Although the duke was checked by a doctor, he did not go to hospital.