A cache of 64 intimate love letters written by Princess Diana to her former lover, James Hewitt, is being offered for sale in the United States, with a price tag of up to $1 million (£780,000). The potential sale has raised concerns that the highly personal correspondence could be made public, causing embarrassment to the royal family and prompting some to label the move as Hewitt's "final insult" to Diana's memory.

The letters, penned between 1989 and 1991, including the period when Hewitt was serving in the Gulf War, are said to be in Diana's distinctive script and of a highly intimate nature. Ingrid Seward, author of the royal book "My Mother and I," commented on the situation, stating, "James Hewitt has betrayed Diana in so many ways so this is just the final insult. I presume Hewitt desperately needs the money and he sees the value in these letters. It is unthinkable these notes for such a sensitive time in her life could become public."

In October last year, intermediaries on behalf of Hewitt reportedly contacted top London auction house Bonhams with a view to selling the trove. Experts at the firm valued the letters, many of which were sent on distinctive military paper known as "Blueys," at between $400,000 (£311,000) and $600,000 (£466,000). However, Bonhams declined to sell them and advised seeking a buyer in the United States, stating, "We would recommend sale in the US because there do seem to be more potential buyers at this level there, and because the sale is likely to be less contentious than in the UK."

Rival London auction house Sotheby's was also approached about the letters the month before but declined to estimate their worth. Subsequently, US auction house Heritage, based in Dallas, Texas, was contacted. An email sent in late January by a Heritage chief suggests that the best way forward was to work with a potential buyer they already had in mind for a private sale, removing the letters from appearing at public auction and distancing the auction house from any negative fallout.

When approached by The Sun on Sunday, a spokesman for Hewitt claimed, "Heritage Auctions were approached on behalf of a representative of James Hewitt solely to value and authenticate the letters for personal reasons and insurance purposes. He has no intention to sell and is not soliciting offers to buy." However, the email from Heritage Auctions discusses arrangements for "the material to be fully insured at $1 million."

This is not the first time Hewitt has been accused of trying to cash in on the documents. In 1999, in a deal with the Spencer family's lawyers, Hewitt promised to store the letters in a safe place, never to be sold, and to be destroyed when he died. In 2000, he was involved in a newspaper sting where it was revealed he had suggested selling the mementos for £10 million. Three years later, he admitted live on CNN that he wanted to sell the letters for the right price, claiming they were "important historical documents" and "nothing to be ashamed of."

The written words in the letters are the copyright of Diana's estate, managed by her sisters and other family members. Legal experts have said that copyright would not be breached if the letters were displayed in a museum or exhibition rather than published.

Princess Diana, who first met Hewitt at a party hosted by her lady-in-waiting in 1986, confirmed the relationship in her 1995 BBC interview with Martin Bashir. She officially divorced from the then Prince Charles in 1996 before her tragic death the following year in a car crash in Paris alongside her partner Dodi Fayed and their driver, Henri Paul.