The letter, published in part by Britain's Mail on Sunday newspaper, was reportedly written in August 2018, three months after Markle married Prince Harry in at Windsor Castle in England.
Markle's father, who was originally meant to walk his daughter down the aisle, ended up not attending after his interaction with paparazzi photographers was exposed.
INSIDER contacted Kensington Palace about the letter, but officials declined to comment on the contents of the letter, or whether it was actually written by Markle.
But the distinctive cursive writing published in the Mail on Sunday squares with previous examples of her handwriting.
It said: "Your actions have broken my heart into a million pieces not simply because you have manufactured such unnecessary and unwarranted pain, but by making the choice to not tell the truth as you are puppeteered in this."
The letter alleges that Markle senior lied when he said he had no way of contacting his daughter, and also lied when he said that she never helped him financially.
"I called and texted... I begged you to accept help we sent someone to your home... and instead of speaking to me to accept this or any help, you stopped answering your phone and chose to only speak to tabloids."
The letter also said that Prince Harry had been "nothing but patient, kind, and understanding with you," and that hearing Thomas Markle's "attacks" on Harry in the media was "perhaps the most painful of all."
Thomas Markle's frequent media appearances have upended royal convention, which usually sees all new information about the royals tightly controlled.
The letter also said that Thomas Markle was "xated and clicked on the lies" that the press wrote about Meghan, "especially those manufactured by your other daughter, who I barely know."
This refers to Samantha Markle , Meghan's half-sister, who has repeatedly threatened Meghan in the media, saying that if their father's health suffers it will be her fault.
The letter said: "If you love me, as you tell the press you do, please stop. Please allow us to live our lives in peace. Please stop lying, please stop creating so much pain, please stop exploiting my relationship with my husband."
"The only thing that helps me sleep at night is the faith and knowing that a lie can't live forever," it added.
Read the published extracts of the letter below:
It is with a heavy heart that I write this, not understanding why you have chosen to take this path, turning a blind eye to the pain you're causing.
Your actions have broken my heart into a million pieces not simply because you have manufactured such unnecessary and unwarranted pain, but by making the choice to not tell the truth as you are puppeteered in this. Something I will never understand.
You've told the press that you called me to say you weren't coming to the wedding that didn't happen because you never called.
You've said I never helped you nancially and you've never asked me for help which is also untrue; you sent me an email last October that said: If I've depended too much on you for nancial help then I'm sorry but please if you could help me more not as a bargaining chip for my loyalty.'
I have only ever loved, protected, and defended you, offering whatever nancial support I could, worrying about your health... and always asking how I could help. So the week of the wedding to hear about you having a heart attack through a tabloid was horrifying. I called and texted... I begged you to accept help we sent someone to your home... and instead of speaking to me to accept this or any help, you stopped answering your phone and chose to only speak to tabloids.
If you love me, as you tell the press you do, please stop. Please allow us to live our lives in peace. Please stop lying, please stop creating so much pain, please stop exploiting my relationship with my husband... I realise you are so far down this rabbit hole that you feel (or may feel) there's no way out, but if you take a moment to pause I think you'll see that being able to live with a clear conscience is more valuable than any payment in the world.
I pleaded with you to stop reading the tabloids. On a daily basis you xated and clicked on the lies they were writing about me, especially those manufactured by your other daughter, who I barely know you watched me silently suffer at the hand of her vicious lies, I crumbled inside.
We all rallied around to support and protect you from day 1, and this you know. So to hear about the attacks you've made at Harry in the press, who was nothing but patient, kind, and understanding with you is perhaps the most painful of all.
For some reason you choose to continue fabricating these stories, manufacturing this ctitious narrative, and entrenching yourself deeper into this web you've spun. The only thing that helps me sleep at night is the faith and knowing that a lie can't live forever.
I believed you, trusted you, and told you I loved you. The next morning the CCTV footage came out. You haven't reached out to me since the week of our wedding, and while you claim you have no way of contacting me, my phone number has remained the same. This you know. No texts, no missed calls, no outreach from you just more global interviews you're being paid to do to say harmful and hurtful things that are untrue.
