"While discussing the possibility that Meghan might become linked with the National Theatre after the wedding, Hussey became unexpectedly serious about the couple's future," he wrote.
"'That will all end in tears,' she is alleged to have warned. 'Mark my words.'"
The explosive book claimed that Lady Susan, 83, visited Meghan at Nottingham Cottage to offer advice in the lead-up to their wedding.
WATCH BELOW: Prince Harry and Meghan, Duchess of Sussex kiss on stage
The Duchess of Sussex then allegedly became "insistent" that she would not let the Royal Family dictate her actions and thoughts.
Furthermore, Tom claimed the Queen was relieved that Meghan didn't attend Prince Philip's funeral in April last year.
"Thank goodness Meghan is not coming," he reports the monarch saying "in a clear voice to her trusted aides."
The explosive claims by Tom, who is best known for his unauthorised biographies of British celebrities, comes a week after Meghan and Prince Harry visited New York City.
The couple flew in to attend the United Nations General Assembly to mark Nelson Mandela International Day where Prince Harry made a keynote speech.
The Duke of Sussex spoke about the South African icon's life and legacy and touched on a photograph he owns of his mother, the late Princess Diana, with Mandela.
"On my wall, and in my heart every day, is an image of my mother and Mandela meeting in Cape Town in 1997," he said.
Whilst Meghan didn't speak on the UN stage, she and Harry did pose for photos with key figures including the President of the United Nations General Assembly and Deputy Secretary General of the UN, Amina Mohammed.
This story first appeared on our sister site Now To Love.