The celebrity title won the UK and Spanish rights to print images of Jolie and Pitt's baby for an undisclosed sum, following a bidding war with the magazine's rival titles.
The couple agreed to sell the images of baby Shiloh Nouvel Jolie-Pitt to picture brokers Getty with the intention of donating all the proceeds to charity. Sources in the industry suggest the payment could be as much as £3.75m.
Hello! and Getty are now planning to sue websites that attempt to show any of the images, after American gossip blogs Oh No They Didn't and Defamer published images of Jolie sleeping beside her baby while Pitt looks on.
The images of baby Shiloh will be published by Hello! tomorrow, with US magazine People following the next day.
The source of the leak has yet to be discovered.
In a strongly worded statement, Hello!'s features editor Juliet Herd, said the magazine had instructed its lawyers to investigate what it described as "illegally obtained" copies of images, which were a "flagrant abuse of copyright".
Jolie and Pitt's daughter was born on May 27 in Namibia under an veil of media secrecy.
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