It comes as police in the Philippines investigate suspected child sex abuse images reportedly found at Mr Markle’s former home.
The Met previously said it had shared the letter with Meghan but did not inform other forces about the accusations.
The force has now said a voluntary referral has been made to the 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 force has also now said it intends to “make a number of referrals to relevant overseas law enforcement agencies in relation to the allegations contained within the letter”.
“That process is under way and will be completed in the coming days.”
Meghan has said she is “deeply distressed” by allegations that her estranged father possessed child sex abuse images.
Mr Markle has denied possessing the images, calling the accusations “sick lies”, in an interview with the Daily Mail.
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