In a judgment on Thursday, Mr Justice Warby ruled that the publication of the letter, which was sent in August 2018, was “manifestly excessive and hence unlawful”.

“It was, in short, a personal and private letter,” he said.

“The majority of what was published was about the claimant’s own behaviour, her feelings of anguish about her father’s behaviour – as she saw it – and the resulting rift between them.

“These are inherently private and personal matters.”

He added: “There is no prospect that a different judgment would be reached after a trial.”

In a statement, the Duchess of Sussex said she was grateful to the courts for holding Associated Newspapers to account “for their illegal and dehumanizing practices”.

“The damage they have done and continue to do runs deep.”

Meghan said her “comprehensive win” means “we have all won”, adding: “We now know, and hope it creates legal precedent, that you cannot take somebody’s privacy and exploit it in a privacy case, as the defendant has blatantly done over the past two years.”

A spokesman for ANL said: “We are very surprised by today’s summary judgment and disappointed at being denied the chance to have all the evidence heard and tested in open court at a full trial.

“We are carefully considering the judgment’s contents and will decide in due course whether to lodge an appeal.”

Media lawyer Mark Stephens said he envisages the newspaper publisher will seek to go to the Court of Appeal “to have a more definitive ruling on what the law is going forward”.

“Essentially this judgement in its widest context puts manacles on the media.”

Meghan is seeking damages for alleged misuse of private information, copyright infringement and breach of the Data Protection Act over five articles published in February 2019, which included extracts from the letter.

The judge made two rulings on the case for copyright infringement.

He found the publication of the letter infringed the duchess’s copyright. But he said the issue of whether Meghan was “the sole author” of the letter or Jason Knauf, former communications secretary to the Duke and Duchess of Sussex, was a “co-author” should be determined at a trial.

The data protection claim was not considered at the hearing in January and is still outstanding.

However, Mr Stephens said that by winning her privacy claim Meghan could say she had been “vindicated” and he expected she would drop the other parts of the case “as soon as she can”.

Meghan, 39, sent the handwritten letter to her father, Thomas Markle, in August 2018, following her marriage to Prince Harry in May that year, which Mr Markle did not attend. The couple are now living in the US with their son Archie, after having stepped back from their roles in the Royal Family.

The full trial of the duchess’s claim had been due to be heard at the High Court in January, but last year the case was adjourned until autumn 2021 for a “confidential” reason.