The Sun reports that the former EastEnders actress cited the Miranda star's adultery as she began proceedings to end their marriage.
Tom, 34, is said to have confessed to a one-night stand earlier this year, leaving Tamzin, 42, 'devastated'.
The couple have two daughters, Florence, four, and Marnie Mae, one.
A source told the Sun that Tamzin had tried 'desperately' to make their relationship work.
"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 mole said.
Tom has now moved out of the family's home in north London.
We spoke to Tamzin shortly after her daughter Marnie's birth. You can read the full interview here.