Fans of HBO's popular drama "The Sopranos" may have to "fuhgeddabout" a fifth season after James Gandolfini, the actor who plays the family boss, filed a lawsuit seeking a release from his contract.

A source close to Gandolfini said the actor believes he is underpaid relative to other small-screen stars.

"This suit is really about Jim [Gandolfini] being paid fair market value, like other stars of network television shows, and he expects it to be settled in a friendly way," the source said.

Gandolfini receives about $400,000 an episode under a deal renegotiated after the first season of "The Sopranos," according to Variety, roughly in line with stars like Martin Sheen of NBC's "The West Wing" but far below the $1.6 million per episode paid to Kelsey Grammer for NBC's "Frasier."

Gandolfini, a two-time Emmy Award winner for his portrayal of conflicted mobster Tony Soprano, alleged in a suit filed in California that HBO violated a clause in his contract when it failed to notify him of a $20 million deal it struck with David Chase, the show's creator.

"This is nothing more than a further renegotiation tactic by an actor in a binding contract," said HBO spokesman Quentin Schaffer.

An end to "The Sopranos," which broke records for a cable show last year by pulling in 13.4 million viewers for its season premiere, would be a huge blow to HBO, the subscription cable channel owned by AOL Time Warner.

"The Sopranos" is HBO's most popular show, and it paved the way for a string of network hits such as "Six Feet Under" and "Curb Your Enthusiasm." The show's fifth season was scheduled to begin shooting in April, a spokesman said.

Network television shows generate advertising revenue, while commercial-free HBO derives its revenue solely from subscriptions. HBO's business model makes it difficult to calculate exactly how much "The Sopranos" -- and Gandolfini -- are worth.

"Unfortunately there's no way to quantify it," HBO's Schaffer said. "But we know it's valuable."

Several other actors on "The Sopranos," including Michael Imperioli, who plays Christopher Moltisanti; Tony Sirico, who plays Paulie Walnuts; and Jamie-Lynn Sigler, who plays Meadow Soprano, have renegotiated their contracts in recent months, according to reports in industry trade publications.

"Gandolfini has always been extremely supportive of his fellow cast members," said a source familiar with the production. "It would seem out of character that he would want to do something to jeopardize a financially rewarding year for his fellow cast."

Gandolfini and "Sopranos" production company Brillstein-Grey were not immediately available for comment.

"Sopranos" star James Gandolfini, with Edie Falco, says he's underpaid at $400,000 per episode. An HBO spokesman says his contract is binding.