The Queen will play an important role in supporting the King throughout his treatment for a form of cancer, it is understood.

The PA news agency also understands Camilla will continue with a full programme of public duties following the shock health announcement.

She has previously been described as the King’s “rock” by her sister, Annabel Elliot, who gave her view of their marriage for a BBC documentary – Charles III: The Coronation Year.

Charles and Camilla heading to church
Charles and Camilla attend a Sunday church service (Joe Giddens/PA)

Mrs Elliot, Camilla’s younger sister, said: “She is his rock, and I can’t actually emphasise that enough. She is somebody who is completely loyal, and she isn’t somebody who has huge highs and lows.”

The Queen’s close friend Fiona, the Marchioness of Lansdowne, also spoke in the documentary about the close bond between Charles and Camilla, who have known each other for more than 50 years, saying they were an “extraordinary team”.

Elsewhere, royal expert Joe Little, of Majesty Magazine, said of the Queen following the latest health announcement: “She is a massive support in so many ways to her husband the King, only last week we saw that she was holding the fort while he was recuperating at Sandringham.”

Camilla has been supporting the King over the course of his recent hospital stay for an enlarged prostate.

Camilla at Windsor Castle reception
Camilla hosting a reception at Windsor Castle (Kirsty Wigglesworth/PA)

She accompanied Charles to The London Clinic in central London on January 26, where he underwent treatment.

The Queen visited her husband every day over his three-night stay in the private hospital, before appearing at his side again on January 29 as he left having been discharged.

The day after the King was discharged from hospital, Camilla was all smiles at a Windsor Castle reception to thank those involved in a book project to celebrate the centenary of Queen Mary’s Doll’s House.

The following day she launched a charity’s centre supporting cancer patients – Maggie’s centre – at the Royal Free Hospital in London.

Camilla at Cambridge visit
Camilla shaking hands with a child whilst departing a visit to the newly opened Meadows Community Centre in Cambridge (Jacob King/PA)

When asked during the visit about her husband’s health, the Queen replied: “He’s getting on, doing his best.”

On February 1, the Queen visited a charity celebrating 850 years of supporting the elderly.

Camilla travelled to Bath to mark the anniversary of the St John’s Foundation, established in 1174 to tackle poverty and poor living conditions in Bath.

For the second day in succession, the Queen gave an update about her husband’s recuperation, saying: “He’s recovering well, thank you”, when asked about his health.

Camilla meeting people in Bath
Camilla meeting members of the public after attending a Service of Celebration at Bath Abbey (Ben Birchall/PA)

The following day, Camilla joked with Strictly Come Dancing star Johannes Radebe who was coaching a class for Royal Voluntary Service (RVS) staff and volunteers at the Meadows Community Centre in Cambridge.

And on Sunday, she accompanied the King as he attended church for the first time since his hospital stay.

The pair walked to St Mary Magdalene Church in Sandringham, Norfolk, accompanied by a member of the clergy.