Lawyers for Meghan Markle on Monday denied she and her husband Prince Harry collaborated with the authors of a recent book about their life together.

The denial came as Associated Newspapers sought to amend its defence against claims it breached her privacy and copyright by publishing extracts of a letter she wrote to her father.

Former actress Markle became the Duchess of Sussex when she married Prince Harry in 2018. The couple earlier this year left frontline royal duties and moved to the United States.

At a hearing in the case at the High Court in London, Associated, which publishes the Mail on Sunday weekly and Mail Online website, applied to amend its defence.

It argued that she "co-operated with the authors of the recently published book 'Finding Freedom' to put their version of certain events".

Antony White, representing Associated, said the book gave "every appearance of having been written with their (Meghan and Harry's) extensive co-operation".

The news group wanted to allege that she "caused or permitted information to be provided directly or indirectly to, and co-operated with, the authors... including by giving or permitting them to be given information about the letter".

It argues it was justified in publishing extracts of the letter in 2019 because five of Markle's friends had spoken about it in an interview with the US magazine People.

But her legal team said extracts of the letter in question and included in the book were "lifted" from the articles under dispute.

"The claimant and her husband did not collaborate with the authors on the book, nor were they interviewed for it, nor did they provide photographs to the authors for the book," lawyer Justin Rushbrooke told the court.

The case is the most high-profile in Harry and Meghan's increasingly bitter war with the media, particularly the British tabloid press.

The court was told that overall legal costs were estimated to be about £3 million ($3.8 million, 3.3 million euros) up to and including trial, which is due to begin in January.

Harry and Meghan's retirement in March came after reports she was deeply unhappy with life inside the royal family and complaints about media intrusion.

The couple, who live with their young son, Archie, in California, recently signed an exclusive deal with the streaming giant Netflix for an undisclosed fee.

They have set up a non-profit organisation to promote education, mental health and well-being.