The Duchess of Sussex broke contact with her father last year, telling him she and the authorities knew he had abused young girls, the BBC understands.
Sources close to the duchess said Meghan warned Thomas Markle in a letter that he would never meet his grandchildren.
It is understood the letter was sent to Markle, 82, while he was in hospital in the Philippines after having his leg amputated.
It has been reported that images of child sexual abuse allegedly belonging to Markle had been found in his former home in the Philippines and UK police in 2019 told Meghan about claims of abuse against him. Markle, who is based in the US, denies all such allegations.
Meghan has been estranged from her father since 2018. He pulled out of his daughter's royal wedding that year, citing ill health.
The letter sent by in 2025 appears to be the end of what has been a troubled relationship between father and daughter.
It is understood details of the letter, first reported in the Sun on Sunday, external, include Meghan telling Markle that he would never meet his grandchildren, Archie and Lillibet, with Meghan making it clear she would never contact her father again.
She also apparently told her father that authorities were aware of the claims made against him.
Philippines officials have confirmed they are investigating the allegations about Meghan's father, telling the BBC on Friday that its anti-human trafficking and child sexual exploitation task force was investigating the matter.
Philippines Interior Minister Jonvic Remulla separately said national police had secured Thomas Markle's computer and would carry out a digital forensic examination.
Meanwhile, Thomas Markle has given an interview to the Mail on Sunday, external in which he acknowledged paying for sex in the Philippines but denies sleeping with underage girls and ever possessing child abuse images.
It comes as the Metropolitan Police confirmed on Saturday it received a letter making a "number of claims" relating to Meghan's father in 2019.
The force said it shared the letter with the Duke and Duchess of Sussex at the time, although it did not pass on the contents to other law enforcement agencies.
The Met said it is now reviewing its handling of the matter.
In a statement shared on Friday, Markle revealed the existence of the letter – saying it contained "disturbing allegations" about her father.
"I am deeply distressed," Meghan wrote. "My thoughts are with any children who may have been harmed."
She said she hoped it could be understood why she had largely cut off contact from her father and did "not allow him to have contact with our children".
Meghan is the only daughter of Thomas Markle and her mother Doria Ragland.
Markle moved to the Philippines last year after he had a series of strokes.
He is reportedly now living in a Florida rehabilitation centre learning to walk again after having his leg amputated.
The Sun reported Meghan's half-brother Thomas Markle Jnr claimed he found a stash of images belonging to his father at his former Philippines home.
He is said to have told police he discovered child abuse videos and pictures among the items. He is understood to have contacted the US Federal Bureau of Investigations (FBI) about it in July.
The FBI has since confirmed it is aware of the allegations.