The King is said to be “greatly encouraged” at returning to public facing royal duties following the positive effect of his cancer treatment.

Charles has been receiving care as an outpatient since early February, and the announcement will be a boost for the monarchy coming to terms with King and the Princess of Wales as cancer patients.

A Buckingham Palace spokesperson said: “His Majesty is greatly encouraged to be resuming some public-facing duties and very grateful to his medical team for their continued care and expertise.”

Sources have stressed that despite the welcome news the King still has cancer and will continue to be treated for the undisclosed form of the disease.

The Palace spokesperson said “… His Majesty’s medical team are very encouraged by the progress made so far and remain positive about the King’s continued recovery.”

To mark the development, Buckingham Palace said Charles and Camilla will visit a cancer treatment centre on Tuesday to meet medical specialists and patients.

The royal event aims to raise awareness of the importance of early diagnosis and highlight innovative research, supported by Cancer Research UK, taking place at the hospital.

The Buckingham Palace spokesperson said: “His Majesty’s treatment programme will continue, but doctors are sufficiently pleased with the progress made so far that the King is now able to resume a number of public-facing duties.

“Forthcoming engagements will be adapted where necessary to minimise any risks to His Majesty’s continued recovery.”

Charles’s diary of events will not be a full summer programme, and attendance will be announced nearer the time and “subject to doctors’ advice”, with “adaptations made where necessary to minimise risk” to the King’s recovery.

It is understood the warmer weather would allow events to be staged outside and lessen the risk posed by other people that many cancer patients face.

Bucks Free Press: King Charles III and Queen Camilla, taken by portrait photographer Millie Pilkington, in Buckingham

The spokesperson added: “The pacing of the King’s programme will be carefully calibrated as his recovery continues, in close consultation with his medical team.”

The Queen is president of cancer support charity Maggie’s, and its chief executive Dame Laura Lee said: “We know at Maggie’s how important continuing to work during treatment can be for many people as it can help to give them a sense of purpose and identity.”

She added: “His openness about his diagnosis has encouraged so many important conversations about cancer and I would like to thank him from my heart for using his own difficult experience to help others.”

A picture marking the first anniversary of the crowning of the King and Queen on May 6 has been released, taken the day after their 19th wedding anniversary on April 10.

Charles and Camilla are shown smiling broadly and walking arm in arm along a path with shrubs and colourful flowers in the background.

Buckingham Palace said in a statement: “His Majesty The King will shortly return to public-facing duties after a period of treatment and recuperation following his recent cancer diagnosis.

“To help mark this milestone, the King and Queen will make a joint visit to a cancer treatment centre next Tuesday, where they will meet medical specialists and patients.

“This visit will be the first in a number of external engagements His Majesty will undertake in the weeks ahead. In addition, The King and Queen will host Their Majesties The Emperor and Empress of Japan for a state visit in June, at the request of HM Government.

“As the first anniversary of the coronation approaches, Their Majesties remain deeply grateful for the many kindnesses and good wishes they have received from around the world throughout the joys and challenges of the past year.”

In January, Charles spent three nights in hospital for a procedure on an enlarged prostate, during which time his cancer – not prostate cancer – was discovered.

After his cancer diagnosis was announced on February 6, the King postponed all public-facing engagement but continued with his duties as head of state behind palace walls, conducting audiences and Privy Council meetings.

The summer months are a busy period for the royal family when colourful set-pieces events like Trooping the Colour, Garter Day and Royal Ascot are held, with the prospect towards the end of the year of overseas travel as Samoa is hosting the biennial meeting of Commonwealth leaders.

A Palace spokesperson said planning continues into ways in which the monarch and his wife may attend events in the summer and autumn although “nothing can be confirmed or guaranteed” and all plans remain subject to medical advice nearer the time.