Dani Brubaker was hired by Spears to take a series of portraits of the star with Sean Preston, 2, and 1-year-old Jayden James in November.
The pictures were printed in last week's edition of OK! magazine without Spears' permission.
Although Brubaker didn't sign a confidentiality agreement and holds the copyright to the photos, sources tell PageSix.com, she breached an agreement with Spears not to make the pictures public.
The Web site says the snapper rejected offers of up to $500,000 for the images in December, claiming her "integrity wasn't for sale," but then hired a manager to strike a deal with OK! magazine.
Spears is expected to hire top litigator Howard Weitzman to sue Brubaker.
A source says, "All legal options are being explored at this time. Brubaker must be dealt with, and Britney will absolutely proceed, in the very least, to sue her."
Brubaker reportedly plans to donate the money she made from the sale of the candid photos to a children's charity.
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