Meghan Markle and Kate Middleton. Photo/Glamour

The Duchess of Cambridge joined the rest of the royal family in sending birthday messages to Meghan Markle via their official social media pages.

The Queen, Kate and William and Prince Charles all shared sweet photos of Meghan, who turned 39 on Tuesday, alongside captions wishing her a happy day.

But royal expert Ingrid Seward, Editor-in-Chief of Majesty magazine, believes that the gesture simply shows that the royals understand how sensitive Meghan is - and claims if she thinks about it, it's actually part of the system she is trying to leave.

Writing for the Sun, she claims that Kate has "better things to think about" than tension with her sister-in-law, but posting a quick snap on Instagram is "not difficult".

She writes: "William is barely speaking to his brother Harry and Kate has every reason to be upset with Meghan but they have risen above it.

"I am sure Meghan’s first reaction will be “what a nice gesture”, then later she might consider it just part of the royal machine she and Harry hate so much.

"We will probably never know but I hope it’s just the former."

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Meghan's birthday comes amid more drama caused by the publication of extracts from a new biography, Finding Freedom.

The Sussexes claim to have had no part in the book, written by Omid Scobie and Carolyn Durand, but many experts find found this hard to believe due to the level of detail.

Among the controversial topics discussed is the relationship between Kate and Meghan, with the authors saying the two women were never close.

They claim Kate did little to bridge the divide between William and Harry, and never really got to know Meghan.

The authors claim Meghan and Kate didn't spend much time together before the Sussexes got married in May 2018 and they didn't get any closer afterwards.

They say that while there were no rows as such, there were some "awkward moments" between the royal wives.

It is claimed Meghan had hoped Kate would reach out to her, but "didn't lose sleep" over it when she did not.

The book says: "Meghan would agree with the assessment that the duchesses were not the best of friends. Their relationship hadn't progressed much since she was Harry's girlfriend.

ALSO READ: 'Frustrated' Prince Harry 'wanted to quit royal family more than wife Meghan'

"But they were not at war with each other, either. There were awkward moments, such as the day the women happened to cross paths at Kensington Palace (in early 2017, when Harry and Meghan were still only dating), and although both were heading out to go shopping - in the same street - Kate went in her own Range Rover."

Andrew Morton, who wrote Princess Diana's tell-all biography, told New Idea that while Kate is putting on a brave face, she is "devastated".

He said: "There’s no doubt the rift between Harry and William has deeply affected Kate. It upsets her."