Joe Biden expressed his gratitude for the words of support that have poured in from across the world, including a private letter from Britain's King Charles, after the former US president announced his cancer diagnosis on Sunday.

"Cancer touches us all," Biden wrote on social media on Monday morning. "Like so many of you, Jill and I have learned that we are strongest in the broken places. Thank you for lifting us up with love and support."

On Sunday, Biden's office disclosed that he was diagnosed with an aggressive form of prostate cancer that has spread to his bones.

The news comes as fresh questions are being raised about the former president's health while he was in office.

"On Friday, he was diagnosed with prostate cancer, characterised by a Gleason score of 9 (Grade Group 5) with metastasis to the bone," Biden's office said in a statement.

"While this represents a more aggressive form of the disease, the cancer appears to be hormone-sensitive which allows for effective management."

A Gleason score of nine means his illness is classified as "high-grade" and the cancer cells could spread quickly, according to Cancer Research UK.

Questions have been raised about how long he might have had this cancer for and if the disease was affecting him while he was still in office.

Biden said his diagnosis was made on Friday after he reported urinary symptoms which led doctors to find a small nodule on his prostate.

In the wake of the cancer diagnosis, many have offered Biden their support including President Donald Trump, former President Barack Obama and former Vice President Kamala Harris.

King Charles has offered his support and sent his best wishes to Biden.

The king, 76, who has met Biden a number of times, is also receiving treatment for an unspecified type of cancer after his diagnosis in 2024.

Biden had then sent his best wishes, saying: "I'm concerned about him. Just heard about his diagnosis. I'll be talking to him, God willing."

Vice-President JD Vance offered well wishes, but questioned whether the American people had a clear picture of Biden's health while the former president was in office.

"We really do need to be honest about whether the former president was capable of doing the job," Vance said on Monday. "And that's that's … you can separate the desire for him to have the right health outcome with a recognition that whether it was doctors or whether there were staffers around the former president, I don't think he was able to do a good job for the American people."

Vance also said he blamed the people around him more than Biden himself.

"This is not child's play, and we can pray for good health, but also recognize that if you're not in good enough health to do the job, you shouldn't be doing the job," he added.

The announcement comes as Biden staves off criticism from a forthcoming book that claims he and his advisers hid his deteriorating health while he was in the White House.

Details revealed last week from the book entitled, Original Sin: President Biden's Decline, Its Cover-Up, and His Disastrous Choice to Run Again, include Biden not recognising actor and frequent Democratic donor George Clooney at a fundraiser last year and aides discussing putting the former president in a wheelchair.

The book will be released on Tuesday.

Nearly a year ago, the former president was forced to drop out of the 2024 US presidential election because of concerns about his health and age.