REBECCA ENGLISH: With Charles heading off for more treatment , William's still unexplained absence shows just how fragile our monarchy is

Elegant in pinstripe and one of her trademark voluminous feathered hats, Queen Camilla led the Royal Family at today’s service of thanksgiving at Windsor for the life of King Constantine of Greece.

Smiling and greeting fellow crowned heads of Europe warmly, Her Majesty’s de facto role as head of the family firm in her husband’s absence was set to be the story of the day.

In her position she would, remarkably, have taken precedence - even without the King by her side - over the heir to the throne, the Prince of Wales, demonstrating just how significantly the royal pecking order has changed in recent years.

But just 46 minutes before the service at St George’s Chapel was due to start, a message from Kensington Palace dropped on my phone as I stood waiting in the castle’s Lower Ward for the royal party to arrive, prompting ripples of concern.

William, his spokesman said, would unfortunately be ‘no longer able to attend the King Constantine Memorial Service this morning due to a personal matter’.

The only two senior working royals present were Queen Camilla and her sister-in-law, Princess Anne at the service today 

Princess Anne, Queen Camilla and Greece's former Queen Anne-Marie (far right) are greeted at St George's Chapel at Windsor Castle today 

It is understood that he personally called the late Greek king’s family to let them know he was unable to attend.

The spokesman refused to be drawn any further on what the matter might be, or when the prince became aware he would have to pull out.

Nor would they say if it had anything to do with his wife’s ongoing medical issues (Kate is not expected to be seen in public until after Easter as she recovers from unspecified abdominal surgery) although they added in response to my question that ‘the Princess of Wales continues to be doing well’.

There is little doubt, though, that the decision was eye-wateringly last minute - normally anathema to the royals, who know how much work goes into organising events such as this and how much their absence will be publicly remarked upon, especially in the current climate.

And it did little to calm the air of jitteriness swirling around our ever-dwindling Royal Family.

I noted that William’s name was even printed in the Order of Service as he had been due to give the second reading - Revelation 21. 1-7 - in honour of his late godfather, the affable, exiled Constantine II, who died in Athens last month at the age of 82.

William’s name was even printed in the Order of Service as he had been due to give the second reading - but pulled out at the last minute due to a personal matter

Of course, there may be a perfectly mundane reason that necessitated the prince to pull out at such short notice (and sources insisted to me - emphasised in capital letters - that there was ‘NOTHING to panic about’).

But William’s last-minute absence occurs against a backdrop of both his wife’s medical issues as well as the King’s shock cancer diagnosis and ongoing treatment, making these worrying days for the Royal Family.

Indeed, it was noticeable to those of us at Windsor today that His Majesty, who has spent the last few days recuperating at the castle following his latest bout of treatment, left for London little more than an hour before the service began for further medical intervention and meetings.

While the nation holds its breath and wishes His Majesty well, we are all also deeply conscious that an increasingly heavy burden rests on the shoulders of William and his wife, not to mention their young family.

It is one made all the more acute by the absence of his former ‘wingman’, Prince Harry, the self-deposed ‘spare’.

Watching our unintentionally slimmed-down family walk into the chapel through the Galilee Porch today only emphasised the fragility of the current set-up.

The only two senior working royals present were Queen Camilla and her sister-in-law, Princess Anne (accompanied by her husband, Vice Admiral Sir Tim Laurence, and her daughter, Zara Tindall, with husband Mike) who are doing a sterling job but are both well into their 70s.

The Duke and Duchess of Edinburgh are currently away.

Other royals present, who are still undertaking limited public duties, were the Duke and Duchess of Gloucester, the Duke of Kent and Princess Alexandra, all of whom are in their 80s.

The elderly Prince and Princess Michael of Kent were also present at the chapel despite the tragic news of the death of their son-in-law, Thomas Kingston, at the age of 45, less than 48 hours earlier.

It would have been a particularly difficult occasion for them given it was the scene of Gabriella and Thomas’s marriage just four years ago.

It is understood that the sad news is not connected to William’s absence.

The other notable royal on parade was the Duke of York, who looked (perhaps predictably) delighted as he strode forwards to lead the family group down from the castle to church on foot.

He was accompanied by his ex-wife, Sarah, reassuringly radiant in grey despite her ongoing cancer battles, and their daughter, Princess Beatrice.

Disgraced Andrew was accompanied by his ex-wife, Sarah, reassuringly radiant in grey despite her ongoing cancer battles 

Andrew's swift assumption of a place at the head of the party did not go unnoticed (Pictured: he reads the hymn sheet at the Thanksgiving Service for King Constantine)

Her Majesty’s de facto role was as head of the family firm in her husband’s absence

While Palace staff were at pains to stress that Andrew was only present as a member of the Royal Family in a private capacity and at the invitation of Crown Prince Pavlos, Constantine’s son, and widow, Queen Anne Marie, his swift assumption of a place at the head of the party did not go unnoticed.

As British royals arrive in ascending level of rank, Queen Camilla was last to enter in the State Bentley for the 11am event.

But that still failed to explain why disgraced Andrew led the rest of the family ahead of her, and may be interpreted by some as yet another attempt to inveigle his way back into the ranks.

The service itself saw 330 guests, many members of European royalty, cram into St George’s Chapel, where Prince Philip - first cousin of King Constantine - is laid to rest, alongside Queen Elizabeth.

The mourners included one king - Felipe of Spain - and three queens, Anne-Marie of the Hellenes, the late King’s window, the glamorous Letizia of Spain, and Noor Al-Hussein of Jordan.

The congregation also boasted two Crown princes and princesses, thirteen princesses, nine princes, three dukes, two duchesses, two infants and too many minor royals and members of aristocratic families to count.

From left to right, Princess Alexia of Greece and Denmark, Carlos Morales, Princess Tatiana of Greece and Denmark and Prince Nikolaos of Greece arrive at the service

They represented a vast array of countries - from Spain to Jordan, Denmark, Bulgaria, Serbia and the Netherlands - and are linked by lineage or position, as well as affection, to the British royals.

It was, undoubtedly, the largest gathering of royals seen in the UK - and almost certainly the world - since the death of Queen Elizabeth and His Majesty’s Coronation.

Indeed, I understand that although the service was organised by the Greek royal family, the invitation to hold King Constantine’s memorial in the royal chapel at Windsor was extended personally by the King.

His wife even hosted a reception for guests afterwards in the Great Hall back up at the castle, although Camilla herself stayed only for the first 15 minutes.

There is no doubt that it would have pained King Charles to miss such a gathering, not least because he was extremely fond of his first cousin once removed, ‘King Con’.

The mourners included three queens, including the glamorous Letizia of Spain 

As a younger man Charles frequently holidayed with him and his sister, Sophia, wife of Spain’s former King Juan Carlos, along with his first wife Diana, Princess of Wales (notably King Juan Carlos, who abdicated from his throne in 2014 amid a swirl of financial and personal scandal, was also present today).

While a sombre occasion, with more than a few tears shed, there was also an air of celebration of the last king of Greece, who ascended the throne at the age of 23 and reigned from 1964 until he was deposed in 1973.

For years, the father-of-five lived in exile in London and his children - Princess Alexia, Crown Prince Pavlos, Prince Nikolaos, Princess Theodora and Prince Philippos - are familiar figures in society circles.

As they returned to the castle, the talk among the party would almost certainly have focused on their memories of the late king of Greece, but also on the health of our present British one - and the absence of his son and heir (for whom the dashing Crown Prince Pavlos stepped in).

Fortunately, I can reveal, William will be seen again in public this week.

He is, I am told, ‘looking forward’ to undertaking his remaining engagements: a sensitive meeting with members of the Jewish community and other faiths at a synagogue as part of a series of visits to highlight the work of organisations trying to bring peace and relief to the Middle East, as well as a number of engagements to mark St David’s Day on Friday.

His return to a more forward-facing role will no doubt bring a crumb of comfort to the millions who wish him well.

But his sudden - and unexplained - absence today amid an abundance of European royals sadly reminds of us how perilously small and fragile our own monarchy now finds itself.