The Mail On Sunday can rely on a recent biography of the Duke and Duchess of Sussex in its defence to Meghan’s High Court privacy claim over the publication of a letter to her estranged father.

Meghan, 39, is suing the newspaper’s publisher Associated Newspapers (ANL) for alleged misuse of private information, breaching the Data Protection Act and infringement of copyright in relation to the publication of parts of a handwritten letter sent to Thomas Markle, 76, in August 2018.

The duchess took legal action last year over five articles published in February 2019, two in The Mail On Sunday and three on MailOnline, which included extracts from the “private and confidential” letter to her father.

At a preliminary hearing last week, ANL asked for permission to amend its written defence to Meghan’s claim to argue that the Sussexes “co-operated” with the authors of Finding Freedom, which was published in August.

The publisher argued that Meghan gave the authors, Omid Scobie and Carolyn Durand, information about the letter to Mr Markle “in order to set out her own version of events in a way that is favourable to her”.

But Meghan’s lawyers said accusations the duke and duchess “collaborated” with the authors was a “conspiracy theory” and argued that references to the letter in the book were simply “extracts from the letter lifted from the defendant’s own articles”.

Ruling on ANL’s application on Tuesday, Judge Francesca Kaye allowed the publisher to amend its defence to rely on Finding Freedom.

The judge said the amended defence did not raise “new defences”, but simply added “further particulars” of ANL’s case.

She added that Meghan “knows the case she has to meet” and that “there is no suggestion that she is in fact unable to do so”.

Judge Kaye refused permission to appeal against her ruling, although Meghan’s lawyers can still pursue a challenge at the Court of Appeal.

In a statement after the ruling, a spokesman for Schillings – the law firm representing Meghan – said: “The court has today stated that The Mail on Sunday will be allowed to amend its legal defence for trial regardless of whether that defence is accurate or true which, based on legally-sworn witness statements refuting the newspaper’s arguments, it is not.

“The Mail has been allowed to prolong this action and try contending its amended defence at trial, where we have no doubt it will fail. This defence has no merit and is in fact false.

“We were prepared for this potential outcome given the low threshold to amend a pleading for a privacy and copyright case.

“The (judge) made clear that if The Mail on Sunday’s defence is indeed weak and without evidence, it would be a house of cards and fall down at trial.”

At the hearing in London last Monday, Meghan’s barrister Justin Rushbrooke QC said: “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.”

He added that neither Meghan nor Harry spoke to either of the authors, who he said “were not given the impression that the claimant wanted the contents of the letter to be reproduced in the book”.

In a witness statement before the court, Mr Scobie said: “Any suggestion that the duke and duchess collaborated on the book is false.

“They did not authorise the book and have never been interviewed for it.”

In a witness statement before the court, Meghan’s solicitor Jenny Afia described much of the information about the Sussexes in Finding Freedom as “extremely anodyne”, “the product of creative licence” or simply “inaccurate”.

But Antony White QC, representing ANL, said that the book gave “every appearance of having been written with their (Meghan and Harry’s) extensive co-operation”.

He said ANL wanted to amend its defence to allege that Meghan “caused or permitted information to be provided directly or indirectly to, and co-operated with, the authors of the book, including by giving or permitting them to be given information about the letter”.

The publisher’s lawyers also argued that Meghan discussed the letter to Mr Markle with the Kensington Palace communications team before she sent it because she wanted to use it “as part of a media strategy”.

Meghan’s case against ANL, which has been listed for a 10-day trial in January, is one of several High Court claims recently brought by the duke and duchess against media organisations.

Harry brought cases against News Group Newspapers and Mirror Group Newspapers over alleged historical phone hacking last September, just two days before Meghan filed her case against ANL.

Meghan is also suing the Splash News and Pictures Agency over “long lens” photographs of her and her son Archie taken in a Canadian park in January this year.