Mr Hewitt, who had an affair with the princess, said she wrote 64 letters addressed to him.

He claims he has been "wrongfully deprived" of the letters, which are in the hands of solicitors, Lawrence Graham, acting for Princess Diana's estate.

Joshua Rozenberg: Mr Hewitt says he hopes the case does not reach court

Michael Coleman, senior partner of Harkavys, Mr Hewitt's solicitors, said his client did not want to take legal action, but was left with no choice.

"Mr Hewitt deeply regrets that he has been left with little alternative," he said.

"He has asked me to say he very much hopes that it will not be necessary for there to be a full public trial at which the ownership of the letters needs to be tested and that those in possession of them will behave lawfully."

Love letters on the move

The letters were sent between 1989 and 1991 at the height of the affair between the former Life Guards officer and Princess Diana.

They are said to show a princess deeply in love with the man she later admitted to having "adored".

The correspondence was allegedly taken from a safe in Mr Hewitt's home while he was on a trip to Spain.

Diana: Adored James Hewitt

The former fiancee of Mr Hewitt was arrested at Waterloo station in April amid allegations that she tried to sell the letters to The Mirror newspaper for £150,000.

Former model Anna Staiano Ferretti, 39, was questioned by officers at Charing Cross police station but released on bail. No charges were brought.

The Mirror subsequently handed the letters to the late princess's private office.

Editor Piers Morgan said in a statement: "The Mirror shares the belief of Diana's family that the safest place for these letters to reside is with them and not with Major Hewitt."

The princess's mother Frances Shand Kydd said she and the other executors of her daughter's will would be happy to hand the letters back provided Mr Hewitt guaranteed their "safekeeping".

Copyright questions

However, a copyright expert has claimed that even if the letters were returned Mr Hewitt would be unable to sell them for publication - but could sell them privately to collectors.

Mr Hewitt says he was "wrongfully deprived"

Alan Williams, head of Media and Technology at solicitors Denton Hall, compared receiving a letter to buying a painting.

"If you buy a painting from an artist you buy the canvas, frame and oils but you do not buy the copyright," he said.

"You could argue that when the princess wrote the letters she was giving him the paper and the ink but not the copyright."

As a result Diana's estate might be able to ban the letters from being sold for publication.

He added that even if the estate was forced to hand over the documents it might legally be able to smudge the ink to make them unreadable.

"It is an interesting point as to whether he is entitled to the letters un-rearranged or he just owns the paper and ink," he said.