LIEV SCHREIBER’s new Showtime series, “Ray Donovan,” has taken off like wildfire, but his fiancée Naomi Watts wants nothing to do with it!

Pressured by producers at the cable network, a source says the hunky 45-year-old actor has been trying to convince pretty Naomi to appear on the show, but she flat-out refused.

“Naomi’s made a point of distancing herself from Liev’s career, and this is no different,” a friend told The ENQUIRER. “She doesn’t like to mix business with pleasure. Besides that, she knows Liev’s show doesn’t need her help.”

THE show’s deb ut on June 30 drew a Showtime record for an original premiere of 1.35 million viewers – and the second episode fared even better.

In the series, Liev plays the title character, a “fixer” who handles crisis situations for L.A.’s rich and famous. Jon Voight (Angelina Jolie’s dad) co-star’s as Ray Donovan’s father.

Meanwhile, Liev and Naomi, who have two young sons together, agreed years ago that they would no longer get involved in one another’s careers, according to the source.

But “Liev’s producers love the idea of having Naomi on the show,” said the friend. “Now he’s upset that Naomi won’t bend the rules.”

Liev is asking a lot of Naomi, 44, because her schedule is tight. The beauty’s biopic “Diana,” about the late Princess of Wales, is due out later this year – and she has four other films in the works.

“Naomi’s not going to budge,” added the friend. “She believes working together on the show would kill their romance – and she thinks Liev will thank her one day for turning him down.”