ASHTON KUTCHER plunged the final knife into the heart of his estranged wife Demi Moore – he filed for divorce so he can finally marry Mila Kunis and have the child he’s always wanted.

But an insider says it was Mila’s ultimatum that spurred him to file on Dec. 21.

“Mila was sick and tired of Ashton dragging his heels,” revealed a source close to the couple. “She finally told him if he didn’t divorce Demi she’d kick him to the curb.

“But Ashton still waited until Demi was out of the country before filing the papers.”

Kutcher, 34, cited irreconcilable differences and indicated in the documents that he wasn’t seeking spousal support – nor is he asking the court to deny Demi any.

“Basically, he’s open to giving her something just to get rid of her,” said the source.

As The ENQUIRER reported in our Dec. 17 issue, Demi, 50, has been on a desperate search for love and reportedly hooked up with much younger men, in­cluding art dealer Vito Schnabel, 26, and New Zealand-based actor Martin Henderson, 38.

Sources said she even tried to woo back one-time lover Leonardo DiCaprio, 38, but he wanted nothing to do with her.

Ashton and Demi wed in 2005 and split last November following reports of his cheating with several women.

Meanwhile, Ashton has had a crush on Mila, 29, ever since they played boyfriend and girlfriend in “That ’70s Show,” beginning in 1998.

The pair was recently spotted shopping for Christmas presents in his home state of Iowa, where they’re expected to spend the holidays with his family.

“Who knows?” said the source. “They may make their announce­ment SOON.”