"In an explosive open letter, Meghan's estranged relative slammed the wedding as a "fake fairy tale".
The man who shares a father with the Suits star addressed the 33-year-old royal with: "Dear Prince Harry, It's not to late, Meghan Markle is obviously not the right woman for you.
"As more time passes to your Royal Wedding, it becomes very clear that this is the biggest mistake in royal wedding history.
"I'm confused why you don't see the real Meghan that the whole world now sees.
"Meghan's attempt to act the part of a princess like a below C average Hollywood actress is getting old."
The letter was published in In Touch Weekly after Markle Jr wrote it on April 26, just weeks ahead of his half-sister's big day.
After slamming her relationship he later wrote a new open letter to Meghan at the beginning of May - sharing fond childhood memories and wishing her well for the big day.
The 51-year-old, from Oregon, claimed Meghan abandoned their father Thomas Markle Sr, despite him pouring money into her career when she first started out.
He wrote: "What kind of person starts out by using her own father until he's bankrupt, then forgets about him in Mexico leaving him broke, over mostly all her debts.
"And when it's time to pay him back she forgets her own father like she never knew him."
The lack of a wedding invite for Thomas also appeared to be a sore spot, as he added: "Not to mention, to top it all off, she doesn't invite her own family and instead invites complete strangers to the wedding. Who does that?"
But he added: 'Meghan is still my sister, she is family. So whatever happens is up to her, whether she wants to forget knowing me or the rest of her family, family comes first.'"
You can see the handwritten letter here as well as The Sun article:
https://www.thesun.co.uk/news/6298014/thomas-markle-jrs-letter-prince-harry-royal-wedding/