Lawyers for the Duke and Duchess of Cambridge will lodge a criminal complaint with the French prosecutor on Monday over topless pictures taken of the Duchess.
St James's Palace says the photographer breached the couple's privacy - as did the French edition of Closer magazine that printed the images.
A spokesman said: "We can confirm that a criminal complaint is to be made to the French Prosecution Department tomorrow.
"The complaint concerns the taking of photographs of the The Duke and Duchess of Cambridge whilst on holiday and the publication of those photographs in breach of their privacy."
More publications plan to print the photographs, but officials say the couple remain focused on their Diamond Jubilee tour, which landed in the Solomon Islands on Sunday.
On what is the penultimate stage of the tour, William and Kate were welcomed to the Queen's Pacific Realm by Island warriors in a traditional ceremony.
They were paraded through the streets of Honiara in an open-top vehicle shaped like a canoe, and attended a Sunday morning service at St Barnabas Cathedral.
As the group of islands has no military, the couple instead inspected an Honour Guard made up of the Royal Solomon Islands Police Force.
It is the first country visited by the couple during their tour where the Queen is head of state.
Back home, a St James' Palace spokesman described a decision by the Irish Daily Star to publish the topless photographs as motivated "by greed".
The royals' condemnation came as the future of the Irish newspaper became uncertain after its co-owner, Northern and Shell, which prints the Star and Express titles in the UK, said it would end its joint venture with the Dublin tabloid in protest at the publication.
Media tycoon Richard Desmond, who used to own a series of adult entertainment titles and is now the chairman of Northern and Shell, said he was "very angry" at the decision to publish the photos, for which he saw "no justification whatever".
The Irish Daily Star editor Michael O'Kane defended their publication, saying the Duchess was no different from Rihanna or Lady Gaga, and acknowledging he wanted to sell more newspapers.
Italy's Chi magazine has also said it intends to publish up to 23 photographs of the Duchess of Cambridge in Monday's edition - the editor has tweeted that "not even a call from the Queen" would stop him publishing the images.