King Charles III's cancer diagnosis could spur a reconciliation with his younger son Prince Harry -- but the rift with his brother Prince William will be more difficult to heal, royal experts said on Tuesday.

The announcement that 75-year-old Charles was suffering from an unspecified cancer prompted an immediate reaction from the disgruntled US-based prince.

He said he had spoken to his father and would be travelling to London.

He touched down in the British capital on Tuesday afternoon, less than 24 hours after the palace made his father's diagnosis public.

The news sparked immediate speculation it could be a catalyst to heal the family tensions that have blighted the start of Charles's reign.

Royal commentator Richard Fitzwilliams, however, described the rift between Harry, also known as the Duke of Sussex, and the rest of the royal family as "very deep".

Since Harry quit royal duties in 2020 and relocated to California, the family has been increasingly fractured by tensions that have seen a complete breakdown in the once close relationship between Harry and heir to the throne William.

- 'Rapprochement' -

Harry and his American wife Meghan Markle criticised his family in a string of high-profile outpourings including a Netflix series and Harry's blockbuster autobiography "Spare".

Claims by Markle, who is mixed race, in an interview with chat show queen Oprah Winfrey that an unnamed member of the family had expressed concerns about the colour of her son Archie's skin before he was born prompted the late Queen Elizabeth II to famously note that "recollections may vary".

An infuriated William also responded that the royals were "very much not a racist family".

"There's no question that the Sussexes were deeply unhappy when they were senior working royals," Fitzwilliams told AFP.

"There's absolutely no doubt that their interview with Oprah and subsequent films, interviews and Harry's memoir, caused a great deal of bitterness."

The situation between the brothers would be "very hard to heal", he said, but the family should now put on a united front.

"The royal family is in this connection just like any other family and it should pull together.

"And what we therefore ought to be seeing is rapprochement, however gradual or however it's handled in the coming weeks and months," he said.

Prince Harry's car was on Tuesday seen being driven from London's Heathrow airport accompanied by two police cars.

- Shattered trust -

"To the relief of courtiers, the Duke of Sussex will apparently be making the journey without his wife Meghan. The question is will he also see William," the Daily Mail's Richard Kay wrote.

Kay said William and Harry, Charles's sons with the late Princess Diana, were not believed to have even "exchanged a word" for many months.

William had in fact given up on rebuilding their relationship, he said.

The 41-year-old William was reportedly particularly angered by Harry's comments in "Spare" that he may not have married his wife Catherine, Princess of Wales, for love.

He is said to have regarded the suggestion as "the lowest of the low".

"While Charles has been torn by the conflict between his sons and has longed for a rapprochement, William is convinced that trust, the basis of any relationship has been utterly destroyed," Kay said.

Another royal watcher, Camilla Tominey of the Daily Telegraph, said she feared the brothers' feud was now too entrenched.

She said sources suggested the brothers' bond following Harry's incendiary remarks in "Spare" was "beyond all repair".

But she held out the hope that they might put their differences aside in order to help their father beat his cancer.

"The one thing they do still have in common is their shared love of their 'darling Pa', the only parent they have left," she said, adding that a royal rapprochement would "certainly help the king on the road to recovery".