JUST as Ashton Kutcher wrapped up his prolonged divorce battle with Demi Moore, he’s now facing a bitter showdown with girlfriend Mila Kunis over a $100 million prenup!

The “Two and a Half Men” star is insisting that Mila sign on the dotted line – and a source says she wants nothing to do with it.

“Ashton wants to protect the massive fortune he’s earned, but Mila is resisting,” explained an insider. “She’s royally insulted because she stood patiently by his side throughout his fight with Demi.

“They’re both so bullheaded, and neither is budging. This money battle is threatening to wreck their romance.”

Ashton, who’s worth more than $100 million, and $150 million woman Demi separated in November 2011 after six years of marriage. But wrangling over money held up a final divorce settlement until November of this year.

“Mila was so relieved when Ashton finally resolved things with Demi, then days later he tells her he’ll only marry her if she agrees to sign a prenup,” said the source. “But Mila is very traditional and feels that a prenup is a bad omen.”

According to the source, Ashton, 35, and Mila, 30 – who co-starred on “That ’70s Show” – were planning on finalizing wedding details over the holidays, but the squabbling over the prenup has forced them to shelve those discussions for now.