Former President Valery Giscard d'Estaing has written a novel about a secret love affair between a French head of state and an unhappy British princess, who bears a striking resemblance to Lady Diana.

The Princess and the President is due to be published next month and is already the focus of fierce speculation over whether it is based on fact or fantasy.

Giscard, who was president of France from 1974-1981, names his fictional French leader as Jacques-Henri Lambertye, while the heroine is Princess Patricia of Cardiff, a town in Wales.

The pair meet at an official dinner at Buckingham Palace and the beautiful princess soon reveals her sad plight, according to excerpts published in Le Figaro newspaper on Monday.

A dozen days before my wedding, my future husband came and told me that he had a mistress and that he had to decided to carry on seeing her after our marriage, says Princess Pat, evoking memories of rows between Prince Charles and Diana.

The book contains a lot of wishful thinking on the author's part, including the fact that President Lambertye wins a second mandate whereas Giscard was voted out of office just weeks before Diana married the Prince of Wales.

But he has fueled gossip that there might be some truth in the tale by writing a cryptic inscription at the start of the book: Promise kept. At the end of the book, he writes: You asked my permission for you to write your story, she told me. I give you it, but you must make me a promise ...

Le Figaro said the book was bound to cause waves.

Fiction or reality? Only the former president holds the key to this troubling story, it said.

Giscard was only 55 when he lost power and photographs show that the dome-headed statesman met Diana publicly in the years following his election defeat. One Reuters picture shows the former Princess of Wales smiling delightedly at Giscard.

Diana died in a car crash in central Paris in 1997 along with her boyfriend, Dodi al Fayed. Media reports said Giscard and his wife were the first people to send flowers to the hospital where her body was taken.