PRINCE William pulled out of a church service at the last-minute today due to a "personal matter" with the Palace confirming Princess Kate is "doing well".
Queen Camilla led a royal party at St.George's Chapel in Windsor Castle for the Thanksgiving service held in honour of Greece’s late King Constantine.
The Prince of Wales informed the Greek Royal family he could not attend the service due to a personal matter.
The late King Constantine of Greece, who died in January last year, was William's godfather.
William was due to give a reading this morning, but instead, Prince Pavlos, the son of King Constantine carried out the honours.
Kensington Palace would not elaborate further but said that Princess Kate, who is recovering from abdominal surgery, is doing well.
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The Princess of Wales, 42, is currently recuperating at home after her planned procedure on January 16.
Kate spent 13 days recovering at the private London Clinic before heading home - and she is unlikely to be seen carrying out any frontline duties until after Easter.
But in a clear sign of her improvement, the Princess joined William and their children Prince George, Princess Charlotte and Prince Louis on a half-term trip holiday on the Sandringham estate in Norfolk.
King Charles, 75, was at Windsor this morning but left before the ceremony began as he continues to bravely battle with cancer following his shock diagnosis.
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He waved to the public as he was driven to Clarence House in London.
Camilla was joined at Windsor today by Her Majesty Queen Anne-Marie, former Queen of the Hellenes and His Royal Highness Crown Prince Pavlos, along with other members of the Greek Royal Family.
The Queen was seen beaming as she talked to guests upon arrival at the historic church.
She then took a seat next to Princess Anne as the memorial service got underway.
Prince Andrew, his ex-wife Sarah Ferguson, Princess Beatrice and Princess Eugenie were also spotted arriving at the special service.
The Queen will also attend a reception with the Greek royals after the service concluded.
The King was said to be close to his second cousin Constantine - who was the former monarch of Greece.
His Majesty was recently diagnosed with cancer after the disease was spotted during this prostate operation.
In a statement at the time, Buckingham Palace said a "separate issue of concern was noted" after Charles spent three nights in hospital last month.
Tests carried out have revealed "a form of cancer", the palace added.
In the meantime, Prince William and Queen Camilla have taken the lead and stepped up to help with royal duties.
Today's service was led by the Dean of Windsor, The Right Reverend Dr Christopher Cocksworth, and attended by
Foreign Royal Families and other guests.
Constantine was a first cousin once removed and sailing partner of the late Duke of Edinburgh.
TIMELINE OF HEALTH WOE
HERE is a date-by-date rundown of the health worries to have struck the royals this year.
JANUARY 16: Kate is admitted to the London Clinic for abdominal surgery.
JAN 17: It is announced that the King is to have treatment for a benign enlarged prostate.
JAN 18: William spends time at Kate’s bedside.
JAN 19: The King flies back from Scotland with the Queen.
JAN 21: It is announced Sarah, Duchess of York, has malignant melanoma, a skin cancer.
JAN 23: Kate’s hospital stay passes one week. JAN 26: The King is admitted to the London Clinic for treatment.
JAN 29: The King is discharged from hospital. Kate leaves the same day.
JAN 31: Queen Camilla announces that the King is “getting on, doing his best”.
FEBRUARY 5: Buckingham Palace announces the King has cancer, but not of the prostate, and will not carry out public-facing duties.
He died at the age of 82 in January last year, decades after being toppled from the throne in a military coup.
It comes after Harry’s recent bombshell interview whereby the Prince revealed his hopes that Charles’ cancer fight will heal the royal rift.
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The Duke of Sussex sat down for a wide-ranging chat for the first time since Charles postponed all public-facing duties.
Harry decided to fly thousands of miles from his California home to be at the King's side - but only met with his dad for 30 minutes.