Meghan Markle "has humiliated her critics and humbled the royal family," a biographer has said ahead of the Duchess's 40th birthday.

Tom Bower believes the former actress's next steps are to conquer the US - and then the world.

He said when she celebrates her 40th birthday on Wednesday she will be 'rightly thrilled' about her 'triumph'.

The biographer described Meghan as one of the world's superstars.

He said that within 18 months of leaving the UK and their roles as working royals behind, she and Prince Harry are living an enviable lifestyle in LA.

Mr Bower cited Netflix, Spotify and book deals in their success.

The biographer described Meghan as one of the world's superstars (

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sussexroyal/Instagram)

In a column for the Sun, the biographer wrote: "She has humiliated her critics and humbled the royal family.

"Her next steps are to conquer America, then the world.

"Critics carp that Meghan’s megalomania has no limits. Her admirers retort that she is purposely fulfilling the American Dream."

As well as a deal with Netflix, the couple have launched a podcast and Meghan has written a children's book since leaving their duties as working royals.

Meghan recently published a children's book (

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PA)

It was also announced that Harry is releasing a tell-all book next year, which is promises to be an "accurate and wholly truthful" first-hand account of his life.

Meghan Markle is reportedly using Oprah Winfrey’s party planner for her 40th birthday bash this week.

The Duchess of Sussex wants Colin Cowie to organise celebrations at her and Prince Harry ’s £11million mansion in Santa Barbara, California.

It is expected to be a low-key affair on their sprawling £11million property with enough privacy to entertain their guests with food and drink sourced from local businesses.

Cowie has planned parties for stars such as Jennifer Aniston, Tom Cruise, Jennifer Lopez and Kim Kardashian.

The Duke and Duchess of Sussex (

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PA)

Harry, 36, has ordered a cake from local baker Posies & Sugar, while on the day guests will enjoy “grazing tables” with locally sourced foods and wines from neighbouring vineyards.

It is expected to be a low-key bash compared to her baby shower in 2019.

A source said: “Megan wants a small gathering. About 65 people are invited, her closest friends and family. Colin was recommended to her by Oprah who always puts on fabulous parties.”

This week’s bash will not break Covid rules.

For Meghan's 36th, she and Prince Harry went to Botswana to carry out conservation work with the charity Elephants Without Borders.

Meghan celebrated her 37th birthday - her first as a Duchess - while she attended the wedding of Prince Harry's friends Charlie van Straubenzee and Daisy Jenks.

Meghan's celebration last year was quiet as she and Harry had just moved into their impressive California mansion.