Her Majesty’s lawyers intervened in the High Court case as the two sides locked horns over one of the final bones of contention.
The Duchess’s former communications secretary, Jason Knauf, also “emphatically” denied having any copyright claim to the letter, landing a final blow to the newspaper’s case.
Their interventions on Wednesday paved the way for Lord Justice Warby to award a summary judgment on the outstanding copyright claim and with it, further costs.
The Duchess successfully sued Associated for breach of privacy and copyright relating to the publication of five articles featuring extracts of the letter in February 2019.
In February, she won a summary judgment, a legal step negating the need for witness evidence, in relation to the privacy claim and the bulk of the copyright claim.
One of the final issues on which both sides disagreed was whether the Duchess was the sole owner of the copyright of the letter, having admitted that she sought guidance from others, including Mr Knauf and her husband, Prince Harry.
Associated suggested that the Duchess sought professional advice because she knew the letter would be made public and that it was intended for use as part of a media strategy to enhance her image. As a co-author, Mr Knauf’s role at Kensington Palace might have rendered the letter Crown copyright.
However, the court heard that Mr Knauf had confirmed in writing, via his solicitors, that despite making a “very minor suggestion” that Meghan include a reference to her father’s ill health, he did not co-write the letter.
As such, he said he had no wish to become a party to ongoing legal proceedings.
The letter said: “Mr Knauf did not draft, and has never claimed to have drafted, any parts of the electronic draft or the letter and would never have asserted copyright over any of their content. In our client’s view, it was the Duchess’s letter alone.”
Lawyers representing the Keeper of the Privy Purse, on behalf of the Queen, also confirmed they “did not consider the Crown to be the copyright owner”.
Andrew Caldecott QC, for Associated, said it was “a matter of regret” that Mr Knauf’s lawyers had not made his position clear before the summary judgment hearing in January.
Mr Knauf was one of the four members of Kensington Palace staff known as the “Palace Four” who had indicated that they would give evidence if the matter came to trial.
The quartet, who were among the Duchess’s closest and most senior aides, had insisted they would remain “strictly neutral” and had no interest in helping either side.
The Duchess had already been awarded 90 per cent of the costs of the first summary judgment application and the judge on Wednesday awarded her the remaining 10 per cent.
The costs for the most recent application will be paid on an indemnity basis, meaning they will likely be higher than if determined on a standard basis.
The Duchess is seeking a proportion of the company’s profits as damages in relation to the breach of copyright.
However, Associated objected to her also seeking an account of profits for damages relating to the claim for the misuse of private information. Instead, it wants to serve a witness statement setting out “general information about how revenues are generated by The Mail on Sunday newspaper and MailOnline”.