Kate Middleton has complained to the Press Complaints Commission over a photograph or her in today's Daily Mirror.
Prince William's girlfriend will learn the outcome of the complaint on or soon after 1 May, Press Gazette has learned.
Middleton, 25, submitted a complaint under clause four of the Editors' Code of Conduct (harassment), which forbids journalists from "questioning, telephoning, pursuing or photographing individuals once asked to desist". It also states that journalists must not "remain on their property when asked to leave and must not follow them".
Middleton is complaining about a story on page 22 of today's Mirror headlined "Look out, Wills here comes an heir-bashing". Middleton is pictured on her way to work, with a caption: "SMOULDERING: Kate leaves for work yesterday".
The PCC said in a statement: "The Press Complaints Commission has today received a formal complaint from Kate Middleton that a photograph of her in today's Daily Mirror was obtained as a result of harassment in breach of clause 4 (harassment) of the code of practice.
"The complaint will now be investigated in the normal way and the commission will not be making any further statement on the subject until the matter has been dealt with."
The PCC's complaints committee next meets on May 1.
Middleton has complained informally through her lawyers Harbottle and Lewis to the PCC several times since she became a prized celebrity target for photographers late last year but this is her first official complaint.
A spokesman for the PCC made it clear that the newspapers could be held to account for the actions of freelancer photographers.
PCC chairman Sir Christopher Meyer was influential in brokering an agreement with News International chairman Les Hinton, who instructed his papers not to print paparazzi pictures of Middleton but to continue as normal when she is at public events. Hello! magazine also agreed to the truce but other newspapers have continued to publish.
In a Press Gazette interview earlier last month Meyer said: "Until the story moves on I think she should be able to get in her car and drive to work without being harassed. When she is coming out of a nightclub with Prince William that is, of course, a different matter."