The Duke of Edinburgh has enjoyed remarkably good health all his life but has been admitted to hospital on several occasions more recently, signalling that the years are finally starting to catch up with him.
Philip, who turns 92 on Monday, is known for his active, healthy lifestyle but has suffered from a number of health problems in the last couple of years.
He spent five nights in hospital being treated for a bladder infection last August, a recurrence of an infection from just a couple of months earlier after being taken ill during the Diamond Jubilee celebrations.
He also spent Christmas 2011 in hospital being treated for a blocked coronary artery.
The Duke appeared in good spirits at the garden party at Buckingham Palace that he attended with the Queen shortly before yesterday's planned admission to hospital for an exploratory operation following abdominal investigations.
He attended the event after missing another function on Monday, but there is not believed to be any link as that was due to him losing his voice.
His last stay in hospital was when he was taken to Aberdeen Royal Infirmary while staying with the Queen at her retreat at Balmoral.
He made the journey of almost 50 miles from the castle in the Cairngorms to the city hospital by road in an ambulance on August 15 to be treated for the bladder infection.
It followed him spending five days convalescing at central London's Edward VII hospital after being taken ill following the Diamond Jubilee pageant along the River Thames.
He celebrated his 91st birthday just a day after leaving hospital on June 9 and his doctors said there was no cause to be ''unduly concerned'' about the infection.
Philip was also successfully treated for a blocked coronary artery after being rushed to Papworth Hospital, Cambridge, with chest pains on December 23, 2011.
He was at the hospital for four nights following a ''minimally invasive procedure of coronary stenting'' which meant he was not able to attend the Royal Family's annual Boxing Day shoot, which he normally leads.
Prior to these health problems, Philip's most recent illness was an uncharacteristic cold in October that year that forced him to pull out of an overnight stay in Italy for the launch of the ARC Green Pilgrimage Network.
He had just completed a busy 11-day official royal tour to Australia with the Queen that saw them visit Perth, Melbourne, Canberra and Brisbane.
Commentators billed the long haul trip as the couple's last to the continent because of their age, but the Palace dismissed speculation it was a ''farewell'' visit.
Previously most of Philip's ailments and injuries have been sports-related.
He suffered arthritis in his right wrist from playing polo and tried to dull the pain with Butazolodin, a drug more usually given to lame horses and recommended by his head groom.
It was reported he later stopped taking it because of the side effects.
In 1961, he broke a bone in his left ankle in a collision on the polo field and in 1963, again playing polo, he suffered a gash to his left arm which needed three stitches.
The Duke was X-rayed in 1964 after a fall from his polo pony when he pulled a ligament in his left shoulder.
He also developed synovitis, a rheumatoid condition of the tendon in the hand, after a polo fall.
Very early in the Queen's reign in 1952, Philip fell sick with jaundice and since then he has been in hospital on a few occasions.
Being a royal has agreed with him and he has kept his lean figure throughout, although his tall stature has diminished as he has got older.
As lean as he was in his 20s, Philip still fits into and uses the same naval uniform he wore on his wedding day.
He underwent surgery in 1967 to remove a cyst from his right wrist, in 1987 to repair a hernia and in 1996 to erase a small benign growth on his nose.
His arthritic wrist eventually forced him to give up polo in 1971 - the year of his 50th birthday - after which he took up horse-carriage driving.
Shaking many thousands of hands at official functions worsened the arthritis and caused him to change his once firm grip to a ''limp-wristed'' style.
The Duke tried a copper bracelet and various homeopathic remedies to help ease the problem.
He does not smoke, having stubbed out his last cigarette shortly before his wedding to the Queen in 1947, giving up virtually overnight.
Over the years the Duke has been an advocate of healthy eating combined with exercise.
As he turned 70, one of the Queen's former physicians described him as ''astonishingly fit for a man of his age'' and he has continued to exercise, swimming regularly.
He once said that he more or less follows the Atkins diet and he drinks only moderately.
It was only when he reached the age of 82 that Philip decided for the first time not to take part in the Trooping the Colour ceremony on horseback. Instead, he travelled in a carriage with the Queen.
He was said to have found previous ceremonies so painful he had to lie on the floor afterwards to recover.
The same year, the Palace was forced to deny claims the Duke was suffering from prostate cancer.
Those who suggested he was in poor health were given short shrift.
''Do I look bloody ill?'' he shouted at one estate worker at Sandringham.
Accidental mishaps sometimes left the Duke looking the worse for wear.
In 2005, he was seen sporting dark glasses and a badly bruised left eye after slipping in the bath and catching the side of his eye with his thumb.
In May 2006, the Duke pulled out of a royal engagement after suffering from a trapped nerve in his neck.
After a weekend recovering, he attended the Chelsea Flower Show, appearing no worse for wear.
A chest infection laid him low in April 2008 for a number of days and he was eventually admitted to hospital for treatment.
But even with the respiratory problem he walked into hospital and walked out three days later and went on to make a full recovery.
In August 2008, Buckingham Palace took the unusual step of speaking out to deny a report that the Duke had been diagnosed with prostate cancer.
The newspaper involved later apologised, saying it accepted the story was untrue.
In January 2009, 87-year-old Philip missed a string of engagements with a bad back after pulling a muscle while carriage driving.
Soon after, it emerged that the Queen had cancelled a state visit - due to take place in spring 2009.
Buckingham Palace insisted this was due to ''other commitments'', not the monarch's age nor the Duke's health.
In June 2010, the Duke had minor surgery on his left hand just before his 89th birthday to cure Carpal tunnel syndrome - a common condition that causes pain, numbness and a burning sensation in the hand and fingers.
The hand operation forced Philip to cancel an official trip with the Queen to Crewe.
The first public acknowledgement of his advancing years came as he was preparing to turn 90.
The Palace announced the Duke planned to step down as president or patron of more than a dozen organisations ahead of his milestone birthday in 2011.