British police said on Thursday they would refer allegations about the father of Meghan, Prince Harry's wife, to overseas law enforcement agencies, after a review found the claims had not been passed to other authorities when they were first received.

Authorities in the Philippines said last week they were investigating Thomas Markle over allegations he possessed child sexual abuse images.

He has denied the accusations, telling Britain's Daily Mail newspaper they were untrue and "sick lies".

Last week, Meghan, the Duchess of Sussex, said the news was "very distressing" and that she had been informed by British police of a letter with allegations about her father in the months after her wedding to King Charles' younger son.

London's Metropolitan Police said on Thursday it had received a letter in 2019 in which the author said they were raising concerns purportedly linked to Meghan's security but instead contained allegations of offending outside Britain.

"The letter was assessed and subsequently passed to the office of the Duke and Duchess of Sussex for their awareness," Commander Adam Slonecki said. "The details of the allegations in the letter were not shared with other law enforcement agencies." He said an internal review had led to the matter being referred to the police watchdog and that the force was now sharing the allegations with overseas authorities.

"It is our intention to make a number of referrals to relevant overseas law enforcement agencies in relation to the allegations contained within the letter. That process is underway and will be completed in the coming days," Slonecki said.