The letter, shared to In Touch, reads: 'As more time passes to your royal wedding, its became very clear that this is the biggest mistake in royal wedding history.
'Meghan Markle is obviously not the right woman for you.
'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.'
Thomas also claims their father has not been invited to Meghan's special day.
'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,' the letter continued.
'And when it's time to pay him back she forgets her own father like she never knew him.
'It's very apparent that her tiny bit of Hollywood fame has gone to her head.
'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? You and the royal family should put an end to this fake fairytale wedding before it's too late.'
Mr Markle finished with: '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 would think that a royal wedding would bring a torn family closer together, but I guess we're all distant family to Meg.'
This is not the first time Markle has hit out at his sister.
He previously told Daily Mirror, 'She's clearly forgotten her roots. It's torn my entire family apart. Meg likes to portray herself as a humanitarian, a people's person and a charitable person but she is none of those things to her family.
'She is giving the greatest performance of her life. She is acting phoney. I've read that Meg wants to be like Diana.
'Diana was worshipped by everyone in the world. She was loved for the right reasons.
'That's what Meg wants, but I don't think that's going to happen. She's not genuine like Diana.'
Thomas has not seen his half-sister since 2011 when the pair – and Meghan's film producer ex Trevor Engleson, 41 – attended the Los Angeles funeral of their grandmother, Doris.
It is still unclear as to which members of Meghan's family will attend her wedding.