'Joe Underestimated Her': Sophie Turner Ready to Fight Back in Ugly Divorce War

Sophie Turner's custody battle with estranged husband Joe Jonas is ripping a page from Game of Thrones, and it appears an all-out war looms ahead, according to a sensational report.

Insiders close to the star best known for her portrayal of Sansa Stark in the HBO megahit claimed she is ready to fight back after her ex painted her as a "partying, part-time mom" following his bombshell divorce, RadarOnline.com has learned.

"Sophie says Joe underestimated her," an insider told The National Enquirer. "Sophie knows how desperate Joe is to preserve his squeaky-clean rep, but she can shatter it," spilled the tipster, claiming the actress is "livid" with how things have gone down.

Earlier this week, lawyers for Turner and Jonas confirmed the former flames will begin a four-day mediation process to resolve issues in their split and in regards to custody of their daughters, 3-year-old Willa and 1-year-old Delphine.

As for what sparked his decision to call it quits, sources said that a number of factors were at play but Jonas allegedly heard Turner badmouthing him to a pal, leading him to end their four-year marriage.

Jonas is seeking 50/50 custody after filing for divorce in Miami on September 1 with sources close to him claiming that Turner had been out and about far too often, leaving him alone to look after their daughters.

Turner later accused her ex of refusing to hand over the girls' passports so they could return to England with her, citing an international child abduction law.

She is also seeking to have the divorce proceedings held in her native U.K. Jonas called Turner's request to stop the Miami proceedings "particularly concerning and problematic."

Prior to the mediation, both sides agreed their little ones will remain in New York until the matter is resolved.

An insider with knowledge on the situation said that "Joe looks miserable these days, but he's pushing back just as hard."

In a recent update, her lawyer Stephen Cullen said there had been "some difficulty" between the two parties when it came to scheduling a trial to decide custody of the girls.

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"We are starting mediation tomorrow (Wednesday) and are going full hog to do everything we possibly can to resolve this matter," Cullen added amid speculation Turner may soon return to England after staying at pal Taylor Swift's townhouse.