Meghan Markle has apologised to a UK court after admitting being involved in a favourable biography of her short tenure as a frontline royal in Britain, despite having previously denied it.

The apology came as part of a British newspaper group's appeal against a High Court ruling that it breached the Duchess of Sussex's privacy, by publishing parts of a letter she wrote to her father.

Associated Newspapers, which publishes the Mail on Sunday, Daily Mail and MailOnline, has submitted that she wrote the letter, knowing it was likely to be leaked, despite claiming the opposite.

It is relying on testimony from her former communications adviser, Jason Knauf, to overturn the lower court's ruling that publication was "manifestly excessive and... unlawful".

As part of their case at the Court of Appeal in London, the publishers said Knauf provided information to the authors of a biography, "Finding Freedom". 

Both Meghan and Harry, who quit royal life last year citing media intrusion and moved to the United States, have previously said they had no direct involvement in the book.

But Knauf said in a witness statement that the best-selling book was "discussed on a routine basis" and "directly with the duchess multiple times in person and over email".

She also gave him briefing points about her life to share with the authors, Omid Scobie and Carolyn Durand. Meghan's husband, Prince Harry, was also emailed. 

Knauf told the court in his statement that Harry had told him there should be plausible deniability and Knauf should provide "the right context and background".

That would "help get some truths out there", he is said to have told the aide.

Little Miss Forgetful

In a witness statement made public on Wednesday, Meghan apologised for misleading the court about whether she had provided details to Knauf to pass on to the authors.

"I accept that Mr Knauf did provide some information to the authors for the book and that he did so with my knowledge, for a meeting that he planned for with the authors in his capacity as communications secretary," she said.

"The extent of the information he shared is unknown to me.

"When I approved the passage... I did not have the benefit of seeing these emails and I apologise to the court for the fact that I had not remembered these exchanges at the time.

"I had absolutely no wish or intention to mislead the defendant or the court."

A decision on the privacy ruling appeal is expected at a later date.

British newspapers, many of whom have been criticised by Meghan and Harry for the accuracy of their reporting, gave prominence to the former television actress saying sorry.

The Sun carried a front-page cartoon of Meghan's head superimposed on a Little Miss character, with the headline: "Little Miss Forgetful."