'Why did he put it in there?' Princess Diana's friend RICHARD KAY reveals the one exchange from Earl Spencer's explosive memoir that left him totally 'mystified'

In a special episode of the Daily Mail's Palace Confidential, Richard Kay names the one anecdote from Earl Spencer's new Diana memoir that he simply cannot explain.

Kay, now Senior Editor-at-Large, knew the late Princess both personally and professionally. The pair became friends while he was the paper's royal reporter, and the Princess came to rely on him as a sounding board for dealing with the press.

Extracts from Earl Spencer's incendiary book, Swan Song: Diana, My Sister, have been serialised exclusively in the Daily Mail over the past week, ahead of its publication tomorrow.

The passages prompted a rare rebuke from Buckingham Palace after Spencer claimed that the King, then Prince of Wales, told him 'Rest assured, we'll forget her soon enough!' during a row over Diana's funeral arrangements.

A Palace spokesman said that while it does not comment on books, His Majesty is 'mindful that the pain of fraternal grief can cloud reason, affect judgment and colour memory'.

Speaking to Palace Confidential host Luke Blackall, Kay revealed that he and Spencer spoke several times in the week after Diana's death. What the Earl told him in confidence then, which Kay noted down at the time, chimes closely with claims made in the memoir.

One anecdote in the book, set as Diana's marriage was falling apart, left Kay 'mystified' and questioning the Earl's intentions 

Extracts from Earl Spencer's incendiary book, Swan Song: Diana, My Sister, have been serialised exclusively in the Daily Mail over the past week, ahead of its publication tomorrow 

However, one anecdote in the book, set as Diana's marriage was falling apart, left Kay 'mystified' and questioning the Earl's intentions.

'[Spencer] has been asked to join his sister at San Lorenzo, which famously was Diana's favourite restaurant,' he began.

'They're dining head to head across the table when she suddenly says to him: "Carlos, I'm going to get a divorce."

'There's no date for this. We don't know when it was... But he feels he ought to make an observation to his sister and says: "Well, you've got to think very carefully about this. The public might turn against you."

'And she looks up from the table, fixes him with those huge blue eyes and says: "Carlos, he's in love with his valet."'

Carlos was Diana's childhood nickname for her brother.

A valet is a personal manservant, responsible for laying out royals' clothes, helping them dress and drawing them a bath.

In the book, Spencer writes that his sister never named the member of staff, and that he never asked her why she believed 'such an improbable thing'.

Kay said the remark comes with no context or explanation, and predicted readers would 'instantly speculate' about who the man was.

'I have to say, I'm mystified why he should need to use that anecdote,' he told Palace Confidential.

'They're going to ask, was it true? But I think the main question they're going to have, and I'm afraid it's one I really don't have an answer for, is why did he put it in there?

'My only view is that Charles Spencer, to use that rather well-worn cliche about revenge being a dish best served cold, decided to put it in for that reason.'

Spencer has since defended including the exchange, telling the BBC it came 'in the context of a conversation where Diana is telling me that her marriage is over'.

Watch Kay's full analysis of Earl Spencer's memoir on Palace Confidential, available now on YouTube.