The Prince of Wales' official 70th birthday photograph with his family was a "nightmare" to plan because his sons blew "hot and cold" with their father, a new book has claimed.
Finding Freedom chronicles the Duke and Duchess of Sussex's romance and brief period as members of the monarchy before they broke away to forge a new life for themselves and their son Archie in America.
But the book also comments on how when Charles contracted Covid-19, Harry immediately telephoned his father to find out how the 71-year-old was doing.
Co-authors Omid Scobie and Carolyn Durand wrote in the book, which is published on Tuesday: "'The boys can be hot and cold with their father' disclosed a source, who gave the example of planning the photo session for Charles' 70th birthday, which they called 'an absolute nightmare'.
"'Neither William nor Harry made much of an effort to make themselves available', the source said."
Charles' 70th birthday in 2018 was marked by the release of two official images, featuring the heir to the throne sat on a bench with Prince George on his knee, and sat next to him was the Duchess of Cornwall and Princess Charlotte.
Stood behind the prince and his wife were William and Kate, with the duchess holding Prince Louis, and Harry and Meghan.
The book also claims how the relationship between Charles and Harry was more complicated than a traditional father and son bond.
The authors wrote in Finding Freedom: "'While Charles may be father to Harry, he's also their boss, and that makes the relationship complex for a number of reasons', a source added.
"'Charles is extremely focused on his public image, and there have been times Harry has felt that has taken precedence over everything else', the source said."
The book looks back over the events of the last few years, from when the couple first met and fell in love, their decision to step down as working royals in order to have financial freedom and the first few weeks of the coronavirus lockdown.
But the relationship between Harry and his father is understood to have moved on from the start of the year when the duke sat down to negotiate with the Queen, Charles and William about his future role.
When Charles caught coronavirus in March, the concern aroused in his son is thought to have brought the pair closer.
The book says: "The doctors described the Prince of Wales as in 'good spirits' and his symptoms as mild, it was still enough to fill Harry with worry.
"He immediately called Charles at Birkhall, his Scottish home where he was now quarantined. Harry regularly checked in on his father until he was out of quarantine and recovered – as well as Camilla, who isolated herself as precaution."
A spokesman for the Duke and Duchess of Sussex said: "The Duke and Duchess of Sussex were not interviewed and did not contribute to Finding Freedom. This book is based on the authors' own experiences as members of the royal press corps and their own independent reporting."
Clarence House and Kensington Palace declined to comment.