The Metropolitan police has referred itself to the police watchdog over potential errors in its handling of a 2019 letter warning of dangers to the security of the Duchess of Sussex posed by her father.

The letter made allegations about Thomas Markle, the father of Meghan, who is facing claims of child sexual abuse images offending linked to the Philippines, which he denies.

It was allegedly written by his former wife and the Met says the letter was passed seven years ago to the private office of the Duke and Duchess of Sussex, who were then both working royals.

The Guardian understands that the long letter made claims about Markle’s activities in the US, the Philippines and Thailand.

The Met said it did not investigate as the allegations related to potential unlawful activity overseas.

The key problem for the Met is its admission that it did not share the letter with law enforcement agencies overseas, and that issue has been referred to the police watchdog, the Independent Office for Police Conduct.

The Guardian understands the Met is now sending details of the letter to the FBI in the US and law enforcement in the Philippines, where Markle lived until May, as well as Thai counterparts.

Markle, now in the US, has admitted paying for sex but denies possessing material that would be illegal.

On Saturday Jonvic Remulla, the Philippine interior minister, told the Guardian that all signs point to Markle being a “possible sex offender of the grossest kind” and that a full forensic investigation will give a “definitive picture”.

Officials and investigators in the Philippines are examining digital images alleged to belong to the estranged father of the Duchess of Sussex, and alleged to have been found at his former home in the south-east Asian country.

Meghan’s half-brother, also called Thomas, reported to Philippine authorities last week the alleged discovery of child sexual abuse images.

The allegations about the letter and alleged Met failings surfaced publicly a week ago. The Met has been unable to find its own copy of the letter making the claims, alleged to have been written by Markle’s former wife.

Met Commander Adam Slonecki, who leads its Protection Command, said the letter had been sent in 2019 to the Fixated Threats Assessment Centre: “The author of the letter suggested they were writing to raise concerns relating to the security of the Duchess of Sussex. While the letter did not actually contain any information specifically related to security matters, it did contain a number of claims about alleged offending outside the United Kingdom.

“The letter was assessed and subsequently passed to the office of the Duke and Duchess of Sussex for their awareness. The details of the allegations in the letter were not shared with other law enforcement agencies.”

Slonecki added: “A voluntary referral has been made to the Independent Office for Police Conduct (IOPC) in respect of the decision not to inform other law enforcement agencies, and the decision to pass the letter to the office of the Duke and Duchess of Sussex.”

The Met cannot find a copy of the 2019 warning letter in its systems, and asked the Duchess of Sussex to send the copy she had to assist Britain’s biggest force to better understand its decision making seven years ago, which she did.

An IOPC spokesperson said: “We have received a voluntary conduct referral from the Met police, which relates to the handling of a letter it received in 2019 involving allegations of offending. We are now assessing the referral before deciding what further action may be required.”

Possible options include the police watchdog independently investigating or sending the matter back for the Met to investigate.

The scale of embarrassment for the Met will depend on whether investigators in the Philippines assess that the allegations relating to child sexual abuse images have any credence.