The 43-year-old star relocated to the United Kingdom with her six-year-old son Wiley and 15-month-old daughter over the summer (16), prompting a legal spat with the little boy's father, Peter Loughran.
He had a problem with the move, took the matter to family court and on Wednesday (26Oct16) a Los Angeles judge agreed with Headey's ex-husband and ruled she had violated a custody agreement, mapped out in 2013, by enrolling Wiley into school in the U.K. without seeking Loughran's permission.
The decision is a blow to Lena, who allegedly planned to keep her son in England permanently, insisting the education system in the U.S. is not as good as it is in her native United Kingdom.
However, she has been cut a little slack - she doesn't have to take Wiley back to the States immediately and can complete filming on the seventh season of Game Of Thrones, which is expected to wrap in February (17).
The actress has been ordered to pay for Loughran's travel expenses to and from the U.S. in the interim.
According to TMZ, Lena's former spouse is allowed to visit Wiley 10 days every month.
Headey and musician Loughran ended their four-year marriage in 2011, but the divorce was finalised in 2013 following a lengthy feud over custody and financial issues, which were resolved when they struck a deal to share joint legal custody of Wylie.
Meanwhile, the actress welcomed a second child in July, 2015, but she has yet to reveal the little girl's name or the identity of her father.