Meghan, the Duchess of Sussex, has threatened the Mail on Sunday with legal action under the Copyright, Designs and Patents Act for publishing a private letter she sent to her father, in which she said he had broken her heart “into a million pieces”.

Representatives of the duchess, formerly known as Meghan Markle, have been in touch with the newspaper after its decision earlier this month to publish the handwritten message sent to Thomas Markle in which, according to sources, she complained with a “heavy heart” about the pain her father was causing by giving media interviews.

Meghan complained to her father about “manufacturing this fictitious narrative” about being cut out of her life with Prince Harry, aided by his regular newspaper and TV interviews. Thomas Markle missed last year’s royal wedding after it was revealed he had colluded with a paparazzi agency to stage photos of himself at home in Mexico.

He allowed the Mail on Sunday to publish extracts of the letter earlier this month, saying he was doing so in response to critical interviews given by the duchess’s friends in the US magazine People. The newspaper commissioned a “handwriting expert” to analyse the letter.

Royals have previously invoked the copyright act in an attempt to keep material out of the public domain. The Mail on Sunday was targeted by Prince Charles in 2005 after it published extracts from a journal he wrote for friends during the handover of Hong Kong to China. The Daily Telegraph has previously suggested that Kensington Palace was considering a similar approach over the recent letter penned by the duchess’s father.

It is unclear how the Mail on Sunday will respond to the potential legal action and the letter remains available on Mail Online.

“A letter is a copyright work as it is a literary work,” said Alex Newman, the national head of intellectual property law at solicitors Irwin Mitchell. “As soon as you create a copyright work, you will own the copyright until it expires automatically. This gives you the right to prevent anyone else copying, or issuing to the public, the whole or a substantial part of your copyrighted work.”

The Mail on Sunday printed nine separate extracts from the five-page handwritten letter, which were promoted on the front page as a world exclusive.

Newman said there were exceptions from copyright law for “fair dealing” when reporting current events and reviews, meaning it was possible to publish extracts without the approval of the creator in certain circumstances.

He said the newspaper’s legal justification for publishing could rest on whether it published a “substantial part” of the letter, taking into account both the proportion of the letter that was printed and the importance of the sections that were used.

Buckingham Palace, which looks after the affairs of the most senior royals, has separately written to the Mail on Sunday to complain about the decision to publish photographs in January featuring the Queen on a pheasant shoot on her Sandringham estate, arguing the monarch had a reasonable expectation of privacy when on her private land.

The newspaper has stood by its decision to print the pictures of the Queen carrying dead birds while on a shoot with Princess Anne and the former Formula One driver Jackie Stewart, insisting that the photographs were taken from a public road.

Kensington Palace and the Mail on Sunday declined to comment.