The King has officially let the world know he is back to work after his cancer diagnosis.

Shaking hands with British Prime Minister Rishi Sunak, the King, 75, welcomed him into the Audience Room at Buckingham Palace, marking the return of their weekly face-to-face meetings to discuss official government business.

It marks the first in-person audience the King and Sunak have had this year, after the Palace announced the King was being treated for cancer.

Cameras were allowed to capture the PM warmly greeting the King on February 21 (UK time), saying it was “wonderful to see you looking so well”.

The King replied, jokingly: “It’s all done by mirrors.”

As the pair sat down opposite each other in two identical Queen Anne chairs, the PM added “we’re all behind you, the country’s behind you”.

“You are kind. I’ve had so many wonderful messages and cards,” the King said.

“It’s reduced me to tears most of the time,” the King revealed, as the pair discussed the benefits of the diagnosis raising awareness for the work of cancer charities.

Sunak said that it had been “nice to see the spotlight that it’s shone” on the disease.

“I hear there has been a lot more attention and interest on those main, wonderful cancer charities, many of which I’ve been a patron of for years.”

On February 6, the King began immediate treatment after doctors discovered cancer during an earlier procedure in hospital in January for an enlarged prostate.

The diagnosis came just 17 months into his reign after the death of his mother, Queen Elizabeth, aged 96, in 2022.

His treatments have been as an outpatient, and on February 13 he and Queen Camilla flew by helicopter to Buckingham Palace from their private Sandringham home for a second bout of treatment, and were driven the short distance to nearby Clarence House.

The King and Queen on the Sandringham estate on February 18. Photo: AAP

Over the weekend, the King and Queen looked relaxed as they went to St Mary Magdalene Church on their Sandringham Estate.

And although he has postponed all public-facing duties, the King is continuing with behind-the-scenes work on his red boxes of state papers.

This includes resumption of his weekly meetings with the PM after their last face-to-face encounter on December 13.

The Queen has been carrying out her royal diary of engagements and says her husband was doing “extremely well under the circumstances”.

His eldest son, the Prince of Wales, resumed his official duties on February 7, after taking time away to care for his wife, Kate, after planned abdominal surgery.

He carried out a ceremony to hand out state honours at Windsor Castle and attended a gala dinner for London’s Air Ambulance Charity, which included a chat to Hollywood star Tom Cruise.

The Bank of England has produced a new £10 note featuring the King. Photo: AAP

Meanwhile, banknotes featuring the portrait of the King will enter circulation in the UK from June 5, but there won’t be an influx of the new money.

The King’s portrait will appear on five, 10, 20 and 50-pound polymer banknotes and will be circulated alongside existing notes that feature the late Queen.

New banknotes featuring Charles will only be printed to replace those that are worn, and to meet any overall surge in demand for notes, the Bank of England said.

He becomes the second monarch to feature on the BoE’s banknotes after the Royal Mint began issuing the first coins featuring his profile into general circulation in late 2022.