The British royal couple reacted with "anger and disbelief" after the French magazine 'Closer' printed a series of photographs of Ms Middleton sunbathing in Provence during a short break last week.
They pressed ahead with a series of engagements in Malaysia yesterday but spent every spare minute locked in crisis talks with aides over what they regard as a "grotesque" invasion of privacy.
After flying to Borneo, where the couple should have been preparing for a trek through the jungle, they stayed up until midnight to thrash out the details of the lawsuit, before St James's Palace issued a statement confirming that they would be suing the magazine and its publisher.
It is the first time a senior member of the royal family has sued in a foreign court to protect their privacy, and if the case goes before a judge it will be the first time such a hearing has taken place anywhere in the world.
William, who has always insisted he would not allow his wife to be hounded in the way his mother was, had earlier referred directly to his mother in a statement that likened Closer's actions to "the worst excesses of the press and paparazzi during the life of Diana, Princess of Wales".
The editor of 'Closer' claimed to be taken aback by the "hullaballoo" the pictures had caused, tastelessly adding: "It's as if we had killed Princess Diana a second time!"
Laurence Pieau also boasted that the pictures would "go round the world" and claimed: "We won't be the only ones to publish them."
Last night, a senior royal source said that preventing the pictures appearing anywhere else was part of the objective of taking legal action.
The royal couple were photographed while they were staying at the Chateau D'Autet, owned by Lord Linley, the queen's nephew, during a short break last week ahead of their nine-day tour of the Far East.
'Closer' claimed the pictures were taken from a public road from which the house and adjacent swimming pool were easily visible, but France has some of the strictest privacy laws in the world, which may have emboldened the royal couple as they considered their options.
William and Kate made no attempt to conceal their fury in a statement released by St James's Palace yesterday.
It said: "Their Royal Highnesses have been hugely saddened to learn that a French publication and a photographer have invaded their privacy in such a grotesque and totally unjustifiable manner.
"The incident is reminiscent of the worst excesses of the press and paparazzi during the life of Diana, Princess of Wales, and all the more upsetting to the Duke and Duchess for being so.
"Their Royal Highnesses had every expectation of privacy in the remote house. It is unthinkable that anyone should take such photographs, let alone publish them."
'Closer' published no fewer than 11 topless pictures of Kate, under a headline that read: "World Exclusive: Sun and Sex in Provence . . . incredible pictures of the future Queen of England as you've never seen her before . . . and as you will never see her again!"
It also alleged that she smoked a cigarette, which royal aides suggested was "highly unlikely" as she has never been a smoker.
The pictures, taken just days after the 15th anniversary of the death of Diana, will become available to subscribers to the magazine's iPad edition, which is available worldwide, unless the start of legal proceedings makes the publisher put a block on the electronic version.
The last senior member of the royal family to begin legal proceedings for breach of privacy was Diana, who sued the owner of a gym for selling pictures of her doing thigh-presses taken using a camera concealed in the ceiling.
The case was settled out of court days before it was due to go before judges.
Two years ago, before she became a member of the royal family, Ms Middleton showed her determination to protect her privacy when she took legal action against a photographer who took pictures of her playing tennis in Cornwall on Christmas Day.
The picture agency, which sold the photos to a German magazine, agreed to pay £5,000 (€6,200) damages plus costs, and apologised, before the matter got to court.
Kate is not the only member of the royal family to suffer the embarrassment of topless pictures being published: in 1999, the 'Sun' newspaper printed an 11-year-old picture of the then Sophie Rhys-Jones having her top lifted up as a prank by the TV presenter Chris Tarrant.
The newspaper apologised the next day after Buckingham Palace accused it of "premeditated cruelty". (© Daily Telegraph, London)
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