Thomas Markle was accused of being a child abuser in a letter sent to Scotland Yard in 2018, it has emerged.
The letter, sent to the Met's Fixated Threat Assessment Centre (FTAC), a specialist unit dealing with potential threats to the Royal Family and other high-profile figures, allegedly accused Thomas Markle Snr of sexually abusing a child.
Its author also claimed the 82-year-old had admitted travelling to Thailand and the Philippines to abuse children.
The development comes after the Duchess of Sussex revealed yesterday that she had been made aware of the 'troubling and disturbing' claims shortly after her wedding to Prince Harry and was assured they were being investigated.
However, the Metropolitan Police said on Friday that it was unable to confirm whether officers had ever passed the letter to Meghan, despite her claims that it had been shared with her.
The Sun has reportedly now seen a copy of the letter, confirming its existence after Meghan publicly disclosed the allegations.
The letter is understood to have since been shared with police, who are conducting internal enquiries into whether it was passed to the Duchess in 2018.
Meghan Markle with her father Thomas Markle in 2011. They have been estranged since 2018
The development comes after the Duchess of Sussex revealed yesterday that she had been made aware of the 'troubling and disturbing' claims
Meghan's brother Thomas Markle Jr (above) claimed to have found the child abuse material
In a statement, the Met said: 'We are aware of media reports that officers from the Met's Royalty and Specialist Protection Command were involved in the passing on of a letter in 2018.
'We are undertaking enquiries internally in an effort to confirm this.'
The Daily Mail understands that the Met has not previously investigated Thomas Sr and is not currently probing him.
Although the force has not confirmed whether any information was passed to law enforcement agencies abroad.
One source told The Sun: 'Questions must be asked about whether the Met ever acted on the information in the letter.
'They promised Meghan they were investigating but nothing seems to have been done at all.'
The latest development follows allegations that Mr Markle possessed boxes containing child sexual abuse images at his former home in the Philippines.
His son, Thomas Markle Jnr, has reportedly accused him of possessing the material, claims which Mr Markle Snr vehemently denies.
Meghan said in a statement this morning: 'I am deeply distressed at the news that has come to light in the last 24 hours, and my thoughts are with any children who may have been harmed.
'In 2018, a letter was sent to the Metropolitan Police and Royalty and Specialist Protection. That letter was subsequently shared with me.
'It made troubling and disturbing allegations against my father and I was given assurances that these were being investigated.
'I hope it can now be understood that, despite certain media publications' distortion over the years, any distance from my father and my choice to not allow him to have contact with our children, was for one reason: to protect them.
The letter is believed to have been sent to the Duchess during summer 2018, soon after she married Harry in May that year at St George's Chapel in Windsor Castle (pictured)
Thomas Sr recovering in a hospital bed after surgery in the Philippines last December
Samantha Markle, left, pictured with her father Thomas Sr and brother Thomas Jnr
'I trusted that the authorities were doing the same for other families.
'Although the breadth of what has now been uncovered is more sickening than one could ever imagine, my sincerest hope is that justice will be fully served and that the priority remains the safety and protection of innocent children.'
The 82-year-old is facing 'sickening' allegations that he possessed a horde of child sex abuse material which included videos of girls under 12 involved in sex acts.
Meghan's half-brother Thomas Markle Jr made allegations overnight that he had found a large stash of child sex abuse images on computers and phones in four large storage boxes 'left behind' by Thomas Sr when he moved back to the US from the Philippines earlier this year.
The material is said to have been discovered in his home in the Philippines.
A specialist unit is investigating the claims in the Philippines, while the FBI has also been made aware of the allegations and is thought to be considering its own probe.
It was confirmed today that the Department of Justice (DOJ) in the Philippines is investigating reports involving 'an American national in his 80s' who is accused of leaving child abuse videos and photos in his home in Cebu province.
Mr Markle, a former Hollywood lighting director, has strongly rejected the allegations, describing them as 'sick lies' and 'the worst thing anyone could accuse me of'.
Speaking to the Daily Mail, he said: 'I'm shocked, I'm hurt and I'm very upset that my son is making these accusations against me.'
The Duchess has been estranged from her father since her wedding to Prince Harry in May 2018, with Mr Markle never having met the couple's two children, Prince Archie and Princess Lilibet.