Lady Colin Campbell has claimed Princess Anne has always "objected strongly" to Prince Harry's relationship with Meghan Markle.
The writer, 71, has aired her thoughts on the royal saga in a 38-minute video published to YouTube.
She alleges that Anne, 70, was the "champion" in the Firm of "don't marry that girl" as the royal believed Meghan, 39, to be "unsuitable".
Jamaican-born Lady Campbell - known as Lady C - touched on various topics relating to the Duke and Duchess of Sussex in the video.
She claims senior royals thought the couple's relationship was a "fling" at first - before quickly realising it was much more serious.
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Lady C said that Meghan then "set out to charm" the family and "succeeded" with Prince Charles, 72, and the Queen, 94.
But she claims that the former actress "did not succeed" with "everyone else" - and the Princess Royal "saw her coming a mile off".
Lady C said Anne "objected strongly" as the relationship developed "on several grounds".
She added that "all of which were based upon conduct and character".
Lady C claimed: "Princess Anne was the champion in terms of 'don't marry that girl, she is unsuitable'.
"'She is wrong for us, she is wrong for the country, she is wrong for the job'. Well, it's turned out to be true."
Lady C also alleged that Anne - the Queen's second child and only daughter - thought Meghan was "flagrantly attention-seeking".
"It just absolutely turned Princess Anne off," she said. "Because she couldn't stand Meghan's constant 'uh, uh, me, me me, me'."
Discussing the allegations of racism levelled at the royal family by Meghan, Lady C insisted that the claims are false.
The Duchess of Sussex said in her bombshell interview with Oprah Winfrey that comments were made about her unborn baby, Archie.
She claimed a member of the royal family raised concerns about the colour of the child's skin - but declined to name who.
Lady C alleged in her video that Meghan "weaponised" some of the "objections" to her on the "grounds of colour".
She said: "No one is the guilty party in terms of racism."
And she added: "There were no concerns about Meghan's colour.
"Princess Anne was rightly concerned that if the marriage proceeded and there were children there would be huge problems.
"Not because of Meghan's colour, but because of Meghan's inability and determination to remain unable to appreciate the cultural differences.
"And to actually have respect for the institution into which she was going to be marrying and the family into which she was marrying."
Harry, 36, and Meghan's sit-down with Winfrey, 67, in March rocked the royal family, with the pair making several huge revelations.
As well as the claims of racism, Meghan also said she had felt suicidal and that Kate Middleton, 39, had made her cry in 2018 - changing the narrative of a widely-reported incident.
Buckingham Palace said in the days that followed that the issues would be addressed "privately".
A statement added that Harry and Meghan would always be "much loved".
Prince William, 38, also told reporters that the royal family was "very much not racist" in the aftermath.
Lady C was previously married to Lord Colin Ivar Campbell, son of the 11th Duke of Argyll.
Buckingham Palace declined to comment on the video.
