Kate Middleton Topless Photos in Closer Magazine -- Royal Family FURIOUS, Threatening Legal Action

Kate Middleton is furious over the release of topless pics taken while on vacation with Prince William in France ... and now the Royal Family is threatening to sue someone's ass over it.

The pics -- published by the French magazine Closer -- show Kate taking off her bikini top while hanging out with William last week at a chateau owned by the Queen's nephew, Lord Linley.

Moments after the pic went public, the Royal Family issued a scathing statement ... saying "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."

In the statement, the Royal Fam said the incident is "reminiscent of the worst excesses of the press and paparazzi during the life of Diana, Princess of Wales" ... who died in a car crash while fleeing from the paparazzi in France back in 1997.

The Royal statement continued, "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."

"Officials acting on behalf of their royal highnesses are consulting with lawyers to consider what options may be available to the Duke and Duchess."

"The Duke and Duchess remain focused currently on their tour of Singapore, Malaysia, Solomon Islands and Tuvalu on behalf of the Queen."

Closer magazine has a UK version as well, which IMMEDIATELY DISTANCED itself from its French counterpart, saying it would "like to make it clear that the two publications make entirely independent editorial decisions. In this respect the comments made by the Editor of the French edition which have reported in the media today do not reflect the opinions of Closer magazine UK."

"Closer magazine UK takes its obligations under the PCC Code extremely seriously and would never publish topless images of a member of the Royal family on its cover or otherwise."