Hello! magazine is taking legal action to stop websites publishing a leaked picture of Angelina Jolie and Brad Pitt with their newborn daughter Shiloh.
The official photos of Shiloh Pitt are believed to have sold for $7m
The photo "has been obtained totally illegally," said the magazine's features editor, Juliette Herd.
Jolie and Pitt sold the photos through picture agency Getty, with all proceeds going to charity.
Both Hello! and Getty say they will seek damages from any internet sites which reproduce the picture.
The leaked photo first surfaced on US gossip blogs Oh No They Didn't and Defamer on Wednesday.
Several sites have already removed the picture, which shows Jolie lying beside her sleeping daughter while Pitt looks on from the side.
The resort where the couple stayed was protected from journalists
At a press conference in Namibia where the couple is currently based, Pitt and Jolie told reporters they had no plans to marry.
"There is nothing in the air. The focus is the kids, and we are obviously extremely committed to the children and as parents together," Jolie told a news conference for local journalists at a hotel in the Namibian coastal town of Swakopmund.
"So that kind of says it for us, and to have a ceremony on top of it is nothing."
The leaked image contains Hello! magazine's logo, suggesting the picture comes from the publication's cover.
The publishers say they have yet to discover the source of the leak.
"It's a complete mystery," said Ms Herd. "And we are very concerned at this breach of copyright".
"Somebody from Hello! must have leaked it," said a spokesperson for People magazine, which owns the rights to the pictures in the US.
"I don't know how it got there."
Baby Shiloh Nouvel Jolie-Pitt was born on 27 May in Namibia, Africa, under a veil of secrecy.
Bodyguards and undercover Namibian police officers shielded the celebrity parents from journalists, erecting green cloth screens around the beach resort where they stayed.
A local human rights group accused the security services of using "heavy-handed tactics" to keep the paparazzi at bay.
The first official photos of Jolie and Pitt with their baby daughter are believed to have sparked a bidding war amongst celebrity magazines.
Experts estimate that global rights to the pictures will have raised as much as $7m (£3.75m).
They will be published by Hello! on Thursday, with US magazine People following on Friday.
"We are very proud that our daughter was born here, and we leave with fond memories and definitely hungry to return," Pitt said of the family's stay in Namibia.