Meghan Markle's half sister Samantha has announced a tell-all memoir in which she's promised to "hold nothing back".

Samantha has been consistently outspoken about Meghan joining the Royal family since she announced her engagement to Prince Harry in 2017.

Now, as the Duchess of Sussex prepares to give birth to a new royal baby, her sister has announced the the first part of her book — titled Shadow of the Duchess — will be "out by the end of April".

She told the Daily Star Online: "It is an honest warm and witty and heartfelt memoir, not a negative pamphlet.

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"(But) all positive? Subjective, I suppose, but most intelligent and reasonable people will like it. Let's just say the nasty gossip about her on the internet is negative and by comparison my book is not. People seem to want dirt. My book is not dirt … but it's not fluff either."

The second part of the book will be out in July, weeks before Meghan's birthday.

Samantha has frequently criticised Meghan about her relationship with their father, even going as far as to say she would be to blame if their father dies, describing her as "inhumane".

She said: "Quite frankly I had faith in her on and off to do the right thing by her dad but I'm sick of the bulls**t. Nobody is trying to get close to her but she's not going to ghost, insult, and destroy this family and skip on her merry way without the truth being revealed.

"(I have) never been afraid of the truth because if the truth makes me lose friends then they weren't friends that I wanted to have anyway.

"You've got to read my book to see what things were really like."