The 39-year-old is said to have walked out on Stephen, also 39, following blazing rows between the pair.
It is thought that Stephen left sick Mel alone and in hospital in the UK, while he returned to their home in Hollywood.
A source told the Mirror: "At the moment she is discussing divorce.
"She knows this isn't the easiest option. It's going to be tough. But she knows her marriage is over."
Mel is believed to have gone into hiding over here with daughters Phoenix Chi, 15, and nine-year-old Angel, but is allegedly furious that Stephen has returned to the States with their daughter Madison.
The source continued: "Mel is raging that Stephen thought it OK to go off with Madison like he has.
"She's in bits about this. It's the final nail in the coffin as far as Stephen's concerned."
While Mel was reportedly planning to spend Christmas at her £4million mansion in Hollywood, she's gone into hiding after discovering Stephen is there.
"She just wants to be as far away from him as possible," the source added. "Now she's found herself somewhere quiet to hide away with Phoenix and Angel and keep herself out of the limelight while she plans her future – and that doesn't include Stephen."