In her written submission to court, the Duchess said: “These are private texts regarding deeply personal family matters that I exchanged with him in the context of his providing to me what he himself has described as ‘close personal support’ as a ‘trusted adviser’.

“I am therefore puzzled as to why he has quoted extracts from those texts in his statement rather than exhibiting them, in full, in a confidential schedule to his statement and allowing them to speak for themselves, set in their full context.

“Be that as it may, I believe that they in fact support my pleaded case and undermine the defendant’s case.”

She added: “Subsequent to receipt of Mr Knauf’s witness statement my solicitors asked his legal team to provide them with a complete copy of the text and email communications from which he quoted; I had no access to any of my texts with Mr Knauf as set out below.

“In addition to those identified above there are a number of further passages in these communications which Mr Knauf makes no mention of in his statement, and which I wish to draw to the attention of the court.”

She went on to list a message in which she explained how she “toiled over every detail” of the letter, fearing it could be misleadingly edited. 

Mr Knauf, she said, replied with an encouraging: “Left nothing to chance! That’s the only way through this”.

In a second message, quoted in the Duchess’s witness statement, he told her: “It’s such a strong and clear letter – with just the right amount of emotion. Hope you’re ok after writing it.”

The Duchess also cites her own response to Mr Knauf’s question asking whether he could discuss the letter with Samantha Cohen, the Duchess’s private secretary at the time.

Saying no, the Duchess wrote: “Also I would prefer if this was not shared in any way with Sam Cohen as I initially said.”

In her submission to court, she explained: “That is important because, as the private secretary, Ms Cohen was our most trusted and closest confidant next to Mr Knauf.

“Even so, this letter was so private that I did not want its contents shared with anyone in my work environment despite feeling obliged to make Mr Knauf aware of it.”