Cancer-stricken King was driving away from Windsor Castle as British and European royalty gathered for memorial service to his late second cousin - before arriving back at Clarence House hours later

The cancer-stricken King was being driven away from Windsor Castle as senior members of the British and European monarchies rallied together for a memorial service in honour of King Constantine of Greece.

The monarch, 75, who is undergoing treatment for an undisclosed form of cancer, was staying at the royal residence on Tuesday but left before the service to his late second cousin started.

Just hours later, His Majesty was spotted waving to well-wishers as he arrived at Clarence House in London, where he has been staying as he is treated for cancer.

The King has not been carrying out public-facing duties since he was diagnosed on February 5.

A source said his absence is not in connection to the Princess of Wales' ongoing recovery from abdominal surgery, who 'continues to be doing well'.

King Charles was seen waving to the public as he arrived at Clarence House on Tuesday afternoon

The King had been at Windsor Castle earlier today but left before the memorial service began

Pictured: King Charles III attends the Sunday service at the Church of St Mary Magdalene on the Sandringham estate on February 18

Queen Camilla wipes away a tear at the memorial service for the life of King Constantine 

Pictured: Guests attend a thanksgiving service for the life of King Constantine of the Hellenes at St George's Chapel, in Windsor Castle, Berkshire

Pictured: L-R The Duke of Gloucester, Admiral Sir Tim Laurence and the Princess Royal are joined by (second row left to right) Edoardo Mapelli Mozzi, Sarah Duchess of York, Lady Helen Taylor and the Duke of Kent attend a thanksgiving service for the life of King Constantine of the Hellenes

Pictured: L-R: Queen Anne-Marie of the Hellenes, Crown Prince Pavlos of Greece and Crown Princess Marie-Chantal of Greece attend a thanksgiving service for the life of King Constantine of the Hellenes at St George's Chapel, in Windsor Castle

It comes as a Greek orthodox monk has claimed that King Charles has turned to him for spiritual advice since being diagnosed with cancer.

It's claimed that the King secretly contacted Archimandrite Ephraim, with whom he's said to have been friends with for over 25 years.

The King and the Abbot are understood to have grown close after Princess Diana's death in 1997.

Earlier this month it was confirmed that King Charles III had been diagnosed with a form of cancer after it was discovered during treatment for an enlarged prostate. 

The palace has not confirmed what type of cancer the King has, or what stage it is, but is it understood that it is not prostate cancer and has been caught early. 

No details as to the type of treatment the King is receiving have been released.

The King is famously a workaholic who takes after his late mother in being very hands-on with the business of monarchy.

But Buckingham Palace confirmed that his team of doctors has suggested he suspend all face-to-face royal duties.

King Charles has been working behind-the-scenes while being treated  for cancer by reviewing paperwork, signing documents and holding private meetings - but some may take place via Zoom calls.

The Prince of Wales, who was due to give a reading at his godfather's memorial, called the family to apologise, while Kensington Palace confirmed Kate was 'doing well' as she recovers at home.

Kate will remain away from official royal engagements until after Easter after her surgery.

Prince Andrew was present today with Sarah, Duchess of York and his daughters Princess Eugenie and Beatrice because it was deemed a personal family event.

It is also understood that the Duke of York was attending the service as a member of the British Royal Family and had been invited by the Greek Royal Family

Pictured: L-R Lord Cavendish of Furness, US climate envoy John Kerry and Dame Norma Major

Pictured: Prince Andrew the Duke of York attends a thanksgiving service for the life of King Constantine

Pictured: Crown Prince Alexander of Serbia and Princess Catherine of Serbia

Pictured: Zara Tindall attends a thanksgiving service for the life of King Constantine of the Hellenes

Pictured: Prince Pavlos, Crown Prince of Greece, Prince of Denmark RE, and Marie-Chantal, Crown Princess of Greece, Princess of Denmark (both front) and Prince Philippos of Greece and Denmark (fifth right) attend the thanksgiving service

Queen Noor of Khorndania and Kyril of Bulgaria and Dax Miller and Alexandra von Furstenberg attended the thanksgiving service today 

Pictured: Birgitte, Duchess of Gloucester and Prince Richard, Duke of Gloucester

Pictured: Jackie Stewart arrives to attend a thanksgiving service for the life of King Constantine of the Hellenes

Charles was unable to attend Constantine's funeral in Athens last year because of commitments which included meeting the president of Cyprus. The Princess Royal represented him at the service.

Constantine, who died in an Athens hospital, acceded to the throne at the age of 23 in 1964.

The young king, who had won Olympic gold in sailing, was initially hugely popular.

By the following year he had squandered much of that support with his active involvement in the machinations that brought down the popularly elected Centre Union government of prime minister George Papandreou.

The episode, still widely known in Greece as the 'apostasy' or defection from the ruling party of several politicians, destabilised the constitutional order and led to a military coup in 1967.

Meanwhile, the Princess of Wales is understood to be 'doing well' as she recovers from abdominal surgery.

Earlier this month, The Daily Mail exclusively revealed that Kate joined Prince William and their three children for a half-term holiday to the Sandringham estate in Norfolk.

'Catherine is recovering well,' a friend told Diary editor Richard Eden.

'She was looking forward to a change of scene and will be able to take it easy in Norfolk while the children let off steam with William.'

As news that Kate had been discharged emerged, a Kensington Palace spokesman said: 'The Princess of Wales has returned home to Windsor to continue her recovery from surgery. She is making good progress.

'The Prince and Princess wish to say a huge thank you to the entire team at The London Clinic, especially the dedicated nursing staff, for the care they have provided. 

'The Wales family continues to be grateful for the well wishes they have received from around the world.'