The legal team are seeking to have the French magazine which printed images of Kate without her bikini top withdrawn from sale.
The photos were taken while Prince William and his wife were staying in a chateau owned by Viscount Linley, the Queen's nephew on a break ahead of their tour of South East Asia.
Lawyers will also be asking French prosecutors to bring criminal charges against the photographer who took the snaps.
According to French law the damages could run into tens of thousands of euros and in theory the magazine editor could be sent to jail for a year.
A royal aide told the Sunday Telegraph: "Their thinking speaks for itself in what the Duke has said about the extreme level of intrusion into their marriage and intimate life together.
"They felt that if they didn't take his stance now after that, they never would.
"They were in an isolated house where the nearest public road was over a kilometre away, so they had every expectation of privacy."
If an injunction is granted, it would result in the magazine being taken off shelves, but would only relate to publication of the pictures in France.
Publications in Ireland and Italy, which have also used the photos, will not be stopped by any ruling.
However the speed at which the couple have acted may deter other magazines from running the images.
The photos have not been published in the British media.