Britain's Queen Elizabeth and Prince Philip walk at Broadlands in Romsey, southern England, in this undated photograph taken in 2007.[Photo/Agencies]

LONDON — Britain's Prince Philip will spend a fourth night in hospital Monday, as he recovers from treatment for a blocked coronary artery.

Queen Elizabeth II's 90-year-old husband is making "good progress," but will remain under observation at Papworth, a specialist heart hospital in Cambridge, a Buckingham Palace spokesman said Monday.

The spokesman, who spoke on condition of anonymity in line with policy, said there are no details on when Philip may be released.

Philip underwent a successful coronary stent procedure at the specialist hospital, where he was taken Friday after complaining of chest pains.

"The Duke of Edinburgh continues to make good progress from the procedure. He remains in hospital under observation and in good spirits," the spokesman said.

It was the most serious health scare suffered by Philip, who is known to be active and robust. He has continued to appear at many engagements, most recently taking a 10-day tour of Australia with the queen.

He missed the Royal Family's traditional Boxing Day shooting party on Monday at the queen's private Sandringham estate in Norfolk, an event he usually leads.

Six of Philip's grandchildren, including Princes William and Harry, visited him Sunday in the hospital.