The couple announced the separation on 29 August (13) following seven years of marriage.
Now 42-year-old Outhwaite has submitted legal documents to officially end their union, citing adultery as the reason for the split.
The news comes amid reports Doctor Who star Ellis confessed to a one-night stand while he was living in the U.S. earlier this year (13) to make new Tv movie Gothica.
A source tells Britain's The Sun, "Tamzin has cited adultery, so she will have a huge advantage when it comes to negotiations. Tom's a reasonable bloke and as it's pretty cut and dried they could be divorced within months."
The couple met through mutual friend James MCAvoy in 2005 and wed a year later (06). They have two young daughters, Florence, four, and Marnie Mae, who was born last year (12).