Naomi Watts doesn't want to star in her partner Liev Schreiber's new TV series 'Ray Donovan', despite his pleas.

Liev is said to be very keen for his partner - who he has been in a relationship with since 2005 - to join him in the hit drama in which he plays the titular character, a fixer for a law firm to the rich and famous.

However, she is resisting his wishes so far, with Naomi insisting she wants to keep their personal and professional life separate.

A source said: ''Naomi's made a point of distancing herself from Liev's career, and this is no different. She doesn't like to mix business with pleasure. Besides that, she knows Liev's show doesn't need her help.''

The opening episode of 'Ray Donovan' attracted 1.35 million viewers making it the biggest premiere in Showtime's history. It has already been commissioned for a second series.

Naomi, 44, and 45-year-old Liev - who have two sons, Sasha, six, and Samuel, four, together - are believed to have made an agreement with each other to avoid starring together on screen.

The actress - who can next be seen playing Princess Diana in the hotly anticipated film 'Diana' - is determined to stick to their agreement because she wants to keep the romance alive in her marriage.

The source added to National Enquirer magazine: ''Naomi's not going to budge. She believes working on the show would kill their romance - and she thinks Live will thank her one day for turning him down.''