The Duke of Sussex, 39, flew from California to London on Monday night to reunite with his family. However, his wife Meghan Markle and their two children Prince Archie, four, and Princess Lilibet, two, have not joined him.
Harry's decision to fly home appears to be a wise one but may also indicate the seriousness of Charles' diagnosis.
HELLO!'s Royal Editor Emily Nash explains: "Prince Harry coming over is a huge moment. The King's diagnosis will have concerned the whole family, but especially his sons and it's only natural that Harry will want to spend time with him after so long apart.
"It's about a family coming together during a difficult time, just as any other family would."
Buckingham Palace confirmed on Monday afternoon that Charles has been diagnosed with a form of cancer and has begun a schedule of regular treatments. And while he has postponed public-facing duties for the foreseeable, His Majesty remains "wholly positive" about his treatment.
Charles will continue to undertake state business and official paperwork as usual. After learning of the diagnosis, Charles personally notified both his sons Prince Harry and Prince William. HELLO! understands that Prince William is in regular contact with his father following the news.
The Prince of Wales is said to have grown closer to Charles in the wake of Harry’s controversial interviews, documentary and memoir, which saw him launch a flurry of allegations at his family. It is not yet known whether William will meet his brother during his trip to London, with their long-running rift ongoing.
William will no doubt help his father with royal duties, while Harry is not expected to take up any royal duties.
The Palace's statement said: "His Majesty is grateful to his medical team for their expert care and swift intervention, is wholly positive about his treatment and looks forward to returning to full public duty as soon as possible."
In 2022, the King asked Parliament to add his youngest brother the Duke of Edinburgh and sister the Princess Royal as extra Counsellors of State so they can deputise for him if need be, and the addition was fast-tracked into law.
The legislation did add Anne and Edward to the list, but stopped short of removing Andrew and Harry. However, the House of Lords heard only "working members" of the royal family would be called upon to act as Counsellors of State.
LISTEN: King Charles' health problems and Princess Kate's recovery: