The author of a new biography of Prince Charles that claims his court is so riven by infighting it is known as “Wolf Hall” has hit back at royal aides’ allegations that she has overstated her access.

Clarence House aides have described as “unhelpful” the comparison with Hilary Mantel’s vision of the brutal court of Henry VIII in Catherine Mayer’s book, Charles: Heart of a King, which is published on Thursday. They have alleged “artistic licence” is being used to describe the amount of time Mayer, a senior editor at Time magazine, spent with Charles.

But Mayer on Monday insisted: “My Prince Charles biography is unauthorised and makes no over-claims about access.” She joked that a “meta-author” had apparently “gone round claiming to have written an authorised bio with oodles of special access”.

An unnamed Clarence House aide told the Daily Telegraph on Sunday: “Catherine had an opportunity to ask the prince three questions when she was compiling the Time profile [of Charles published in October 2013].”

“She had about nine minutes with him; to suggest she had 10 minutes would be stretching it. It seems that artistic licence is being used to portray how much access she had. She has attended a number of events since then as part of the royal rota, but with the same amount of access as any other journalist.”

When writing the book, Mayer is believed to have also gone on two walks around the prince’s estate at Dumfries House in Scotland with Charles and other guests. She also attended a dinner at the same venue. All three occasions were off-the-record but add up to more exposure to the prince than the suggestion that she was only with him for nine minutes. It is also understood that Mayer carefully explains in the book her level of access in an effort to be transparent on the issue.

Neither the prince nor Clarence House are believed to have yet read the Mayer’s book, which is being serialised in the Times ahead of publication, but his aides have gone on the attack in advance. They have pledged to challenge anything that is “grossly untrue” and “detrimental to the prince”.

Extracts already published portray the prince as a man afflicted by insecurity, susceptible to bouts of despair and unable to control the feuding of his staff. The book will claim the Queen is concerned that her eldest son’s activism would redefine the monarchy in a way the public will not tolerate.

On Monday, Clarence House continued its combative stance. “This is a book full of opinion and it is not necessarily fact,” an aide told the Guardian. “Some of it may prove to be true but the passage of time will show much is untrue.”

In response to claims of infighting and comparison’s with Wolf Hall, a spokesman for the prince added: “Clarence House employs over 100 hardworking professionals, many of whom have been there for decades and whose work and dedication is appreciated by their royal highnesses.”

The row over the book comes amid growing scrutiny of Prince Charles’s ambitions for his kingship. Last November, a Guardian investigation revealed that he plans to make “heartfelt interventions” in national life when king in contrast to the Queen’s taciturn discretion on public affairs. Mayer’s biography is believed to be the first of four to published in the coming months.

Clarence House’s pledge to challenge any “gross untruths” found in her book comes after a similarly defensive approach towards a two-part BBC documentary about Charles and the media that was due to be broadcast last month.

It was postponed by the broadcaster after Charles’s officials challenged BBC bosses to justify the documentary in terms of whether it met the corporation’s guidelines and problems with access to royal archive footage. Reinventing the Royals has been made by the media commentator and BBC journalist Steve Hewlett, who told the Radio Times before the original transmission date that the prince’s former adviser, Mark Bolland, was dubbed “Blackadder” by princes William and Harry. The documentary had promised an in-depth examination of the shrewd PR tactics used by Bolland to boost Charles’s public appeal after the death of Diana, Princess of Wales in 1997. It is understood the documentary will now be transmitted.