Meghan Markle’s father is trying to “clear his name” amid negative press coverage by defending the Mail on Sunday against her, a PR expert has said.

The newspaper is fighting legal action brought by the the Duke and Duchess of Sussex over its publication last year of a personal letter sent from Meghan to Mr Markle in which she criticises him. The letter was given to the paper by Mr Markle.

The Mail on Sunday published extracts in February which led to the Sussexes launching legal action, with Prince Harry saying he could no longer be a “silent witness to (Meghan’s) private suffering” and attacking parts of the press for “waging campaigns against individuals with no thought to the consequences”.

The Duchess of Sussex claims her privacy, data protection rights and copyright have been breached while the newspaper says publication was in the public interest.

It emerged on Wednesday that Thomas Markle, whose relationship with Meghan is in pieces, is prepared to be a witness for the newspaper’s defence if a court case takes place.

Meghan and Harry are taking legal action against the Mail on Sunday (PA)

The revelation comes a week after the Sussexes announced they wanted to become financially independent and step back from the Royal Family.

Nick Ede, a popular culture and PR expert, fears the “positive move” by the Duke and Duchess of Sussex to change how they are funded and work will be overshadowed by a protracted war with the press.

But he does not believe Mr Markle’s willingness to appear for the newspaper will stop the couple.

“I think they will have to go all the way now as there is no backing down but I think it shows how far the relationship Meghan has with him has been damaged that he is willing to testify against her,” Mr Ede told Yahoo News UK.

Mr Markle’s relationship with Meghan has made repeated headlines in recent years, with the two not speaking and competing claims emerging in the media over who has tried harder to fix it.

“Whatever you think about him, he’s coming to testify to kind of clear his name, and it’s a terrible PR own goal (for Harry and Meghan),” Mr Ede said.

“I don’t think they saw the Mail on Sunday would take it this far.”

The duchess’s half sister, Samantha Markle, has also told the BBC that if Mr Markle is called to give evidence in a court case between the Sussexes and the Mail on Sunday, “he will come”.

Alongside the revelation that Mr Markle is set to defend the Mail on Sunday, the paper’s sister title the Daily Mail also published legal documents which quote text messages from Mr Markle to Meghan.

“This is becoming a bit of a war… I think this has become quite toxic to be honest,” Nick said.

“It is going to feed the fire of human interest in them but it’s also going to feed and fuel people’s attitude towards them, whether it’s positive or negative.

Harry and Meghan have faced negative press coverage (PA)

“It was quite shortsighted of them to think it wouldn’t get this far and it is going much further than they ever thought it would.

“I think now with hindsight and with what they’ve done by leaving, this is adding to this really negative narrative that I don’t think any of them would want, and it’s going to perpetuate negativity rather than the things that I talk about a lot.

“I think it’s a positive move (stepping back from the Royal Family), it’s changing the monarchy, its showing that they want to earn their crust, do good things as well, and I think (the legal action headlines are) going to be a big dark cloud that could’ve been prevented.”

He said there appeared to be no strategy for communication and said the royals need to develop a “central opinion and trajectory into the future”.