The pages, which are highly critical of her parents' marriage, are expected to be published in a new edition of the diary once they have been handed over.

They are still in the possession of a former employee of the Anne Frank Foundation, who says they were given to him by Anne's father.

Anne Frank died in Bergen-Belsen concentration camp in March 1945, aged 15.

Her diary has absorbed tens of millions of readers worldwide with its simple, vivid account of a young girl's daily life in permanent refuge.

The first edition, published in 1947, omitted some of the more earthy observations of Anne's two years in hiding.

It was presumed that the unabridged version, first printed 40 years later, was compiled from the full text.

But a former employee of the Anne Frank Foundation told researchers he had been given five pages by her father, Otto Frank, shortly before he died in 1980.

The Foundation believes they are a rewritten version of some notes she made in February 1944 which her father may have wished to conceal.

A bookcase hid the door to the Franks' hiding place

The Foundation and the Dutch Institute for War Documentation - which owns the original manuscripts - have asked for the pages to be returned, saying that they will be included in a new edition of the diary.

David Barnouw, a spokesman for the war documentation centre, said: "We thought we had a complete edition. Now this pops up. I don't doubt the authenticity of these pages."

David Barnouw talks about the find

But while they believe the pages themselves are real, the two organisations say they seriously doubt the unidentified man's claims that the notes were a present from Anne's father.

"It is highly improbable that Otto Frank made a gift of this original manuscript to this former employee," they said in a joint statement.