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More doubt has been cast on whether Meghan Markle's father will attend his daughter's wedding to Prince Harry on Saturday, as it is reported he is due to have heart surgery.
Kensington Palace declined to comment.
Ms Markle's estranged half-sister said he had faced an "unbelievable stress".
Speaking to Australia's Sunrise morning TV programme from Orlando, Florida, Samantha Markle said she was not sure if Mr Markle would be travelling to Windsor.
"He sent me a message that he was undergoing heart surgery," she said.
"We're all hoping he pulls through this now."
Mr Markle became embroiled in controversy at the weekend, following reports that he staged paparazzi photographs of himself in the run-up to the wedding.
The pictures showed Mr Markle - apparently unaware he was being photographed - in a series of wedding-related activities, including being measured for a wedding suit.
On Tuesday evening, BBC royal correspondent Nicholas Witchell said Ms Markle wanted her father to be there, but her major concern would be for his wellbeing.Image copyright Daily Mail/SOLO Image caption
Kensington Palace has said events are "a deeply personal moment for Ms Markle".
"She and Prince Harry ask again for understanding and respect to be extended to Mr Markle in this difficult situation," the palace said in a statement on Monday.
Mr Markle had been due to meet Prince Harry for the first time this week, as well as the Queen and the Duke of Edinburgh, before walking his daughter down the aisle at St George's Chapel in Windsor Castle.
Samantha, who has not spoken to her famous sibling in three years - and will not be attending the wedding - told Good Morning Britain the photographs had been done with "good intention" and not for money.
She previously admitted the pictures had been her idea, in order to portray a positive image.
By Jonny Dymond, BBC royal correspondentImage copyright Getty Images
People around Meghan Markle are saying she is having to contemplate her father not walking her down the aisle.
The pressure has been absolutely relentless and she and Prince Harry have been concerned for some time about Thomas Markle's mental state.
He seems to have been rather caught out by the astonishing pressure that fell upon him.Whether by design or accident he fell into this photo agency scam.
We have another example of that unhappy flipside of royalty - which is the extraordinary pressure of the media's attention, part of it brought on by the fact the royal family require the attention of the press to maintain their relevance.
The father of the ex-Suits actress lives in Mexico and is a former lighting director who worked on programmes including the 1980s TV show Married with Children and General Hospital, for which he and his team won two Emmy awards.
He and Ms Markle's mother, Doria Ragland, divorced when Ms Markle was six years old.
Mr Markle, who has two children - including Samantha - from his first marriage, filed for bankruptcy two years ago.
Ms Markle has previously said:"It's safe to say I have always been a daddy's girl - he taught me how to fish, to appreciate Busby Berkeley films, write thank-you notes, and spend my weekends in Little Tokyo eating chicken teriyaki with vegetable tempura."Image copyright Reuters Image caption
The wedding will take place on 19 May at 12:00 BST at St George's Chapel in Windsor.
With five days to go, Kensington Palace revealed that Ms Markle will spend her last night before getting married at a luxury Buckinghamshire hotel with her mother, Doria.
Prince Harry will be staying 15 miles away at the Dorchester Collection's Coworth Park in Ascot, with his brother, the Duke of Cambridge....
- ^ paparazzi photographs (www.dailymail.co.uk)
- ^ told celebrity news website TMZ (www.tmz.com)
- ^ TMZ later reported (www.tmz.com)
- ^ in a third report, the website (www.tmz.com)
- ^ Mr Markle had been due to meet Prince Harry for the first time this week, (www.bbc.co.uk)
- ^ Read more from Jonny here. (www.bbc.co.uk)