The government of Belgium threatened yesterday to sue the French publisher of a book making sexual allegations against the Belgian king.

A statement issued by the office of the prime minister, Guy Verhofstadt, called the book an "assault on the dignity of our nation and its people".

The government instructed the justice minister, Marc Verwilghen, "to take the necessary legal steps against the publisher."

King Albert II has issued a denunciation of the book, which has just come out in Belgium and is due to go on sale in France this week, despite a public warning against publication by the Belgian ambassador to France.

The book reproduces purported extracts from an official investigation into the activities of Albert before he took the throne in 1993.

Published by Flammarion, one of France's leading houses, the book is being interpreted by the Belgian media as a calculated attempt to topple King Albert, 67.

Albert has been married to an Italian, Donna Paola Ruffo di Calabria, since 1959 and they have three children. The family has so far been largely spared the media attention lavished on the royals in Britain.

The book's contentious extracts come from an investigation by a Belgian judge.

Belgian protocol dictates that the royal family does not respond to gossip and lurid accusations, and the king has surprised commentators by his statement on Sunday denouncing the book.

It says: "The royal palace notes with indignation Flammarion's decision to publish a book which utters grotesque accusations against our country, even going as far as to implicate the king.

"It is incomprehensible that a firm like Flammarion which was once so respected discredits itself by publishing allegations made by fantasists in search of sensationalism.

"These allegations have been known for a long time and have been found to be completely without foundation by judicial, parliamentary and journalistic investigations."

Flammarion said yesterday that it was unaware of the controversy in Belgium, where newspapers have been scathing about the book and particularly one of its authors, a Luxembourg-based investigative journalist, Jean Nicolas.

"People are talking about it without having read it," its spokeswoman said. "There is a question mark over Albert but he has not been accused."

Mr Nicolas, who wrote the book with Frédéric Lavachery, argues that he has made no accusations against Albert but simply published legal documents that do so.

He says the official inquiry was never completed.

"Rumours about Albert have been circulating for the last 20 years but this is not a rumour. I present the information in its raw form and I had to justify every line in this book."

Mr Nicolas has courted controversy in the past and faced legal action for allegations which he has found hard to substantiate.

The Belgian daily Le Soir said that the new book was based on "the reconstitution and exaggeration of gossip and fantasies."

"This time Mr Nicolas's megalomaniac goal is nothing less than to obtain the head of the king and also of Belgium itself."

But there is of course no such thing as bad publicity, and Nicolas says he is delighted at the fuss.

"It's selling like hot cakes after the palace's press release. Their reaction is a stupid one but I'm not complaining."