Pippa is 'carrying out James Middleton's promise to Princess Kate'

She subsequently stepped back from public life and was not seen again until her appearance at Trooping the Colour in June.

James Middleton, Kate's younger brother, took to Instagram with a personal message and tribute to the Princess of Wales after she announced her cancer diagnosis in March.

Posting a throwback photograph of the pair as children, James wrote: "Over the years, we have climbed many mountains together.

"As a family, we will climb this one with you too."

Pippa Middleton, the middle child of Michael and Carole Middleton, accompanied the Princess of Wales to Wimbledon last weekend in a moving show of familial solidarity.

Gareth Russell, a royal commentator, exclusively told GB News that Pippa's presence was a continuation of James's promise to "climb this mountain" with Princess Kate.

Russell said: “We're going to see more visibly the love and support that the whole Middleton family gives to the Princess of Wales.

“We know that they're a close family. Pippa and Catherine's brother James put up that very moving post after his sister announced her cancer diagnosis, in which he said they had climbed many mountains as a family before and they would climb this one together.

“And certainly that seems to be what they're doing.

"The Princess of Wales has a very close, solid support network, both in her husband's family and also in her own.

“It was really lovely to see Pippa Matthews out with the Princess of Wales and Princess Charlotte.

"I would imagine that, given how close they are, we'll see many more occasions like this.

“But also, I think what's important to remember is that what we see as the public is not the crucial part.

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"We’ve heard from, as I mentioned her brother, James, that this is a family who rallies around in private and in moments of illness.

“Indeed, in any person's life, the support you receive in private is so much more worthwhile and important than public action.

“It's very clear that the Princess of Wales is close to her family and they remain so.”