Meghan Markle's estranged father, Thomas Markle Sr., has admitted that he hired prostitutes ... but denies ever sleeping with anyone underage.

Thomas Sr. told the Daily Mail on Sunday ... "I have never had sex with anyone under the age of 18."

He also told The Mail that he had previously traveled to the Philippines at least twice before living there and "slept with prostitutes" during those trips.

He added ... "I like Asian women but that is not a crime."

After The Mail asked him about an alleged photo of a bearded man sexually assaulting a girl the publication claimed was in the hands of Philippine authorities ... Thomas Sr. said ... "I have never been in a photograph like that."

We reported earlier ... London's Metropolitan Police confirmed yesterday that they sent a letter to Meghan in 2019 with "several claims" about Thomas Sr. ... but admitted that they didn't forward the letter to any other law enforcement agency outside the U.K.

Thomas Sr. told The Mail that he believes Thomas Markle Jr. -- who reported finding the alleged child porn on his dad's property -- may be behind the letter as well.

Meghan spoke out on the letter as well in her own statement last week ... saying that it was the reason she cut off contact between her kids and Thomas Sr.

Thomas Sr. confirmed their lack of contact and told The Mail that he hasn't been given the opportunity to defend himself against the "lies."

He also said that any child abuse images found on his property were planted there.

Thomas Jr. has previously claimed that he felt bound to report what he said he discovered ... now authorities in the Philippines are currently investigating Thomas Sr. over the alleged child porn stash.