Controversial Sydney boxer Anthony Mundine has lashed out at media critics of Bulldogs rugby league star Sonny Bill Williams, calling them "leeches".

Mundine, a former St George Illawarra player who staged his own walkout midway through the 2000 NRL season to pursue a career in the ring, says critics of Williams are behaving like "parasites".

With Williams understood to be stranded in London en route to Paris to play rugby union after walking out on the Bulldogs, his lawyers are preparing to challenge the foundation of the league's regulations, the salary cap.

"I'm 100 per cent behind my brother and 100 per cent in his corner," Mundine said in Wednesday's Sydney Morning Herald newspaper.

While the NRL's 16 clubs have backed the salary cap in a meeting on Tuesday, Mundine says it should be abolished.

Clubs and fans fear some teams may cease to exist if the salary cap is removed, but Mundine argued players should simply be able to earn top dollar.

"If he wants new goals and new dreams, he'll inspire kids to chase their goals and dreams, and everybody should support him," Mundine said.

Mundine said there were "lots of haters out there" and described media critics as "nothing but leeches and parasites as far as writing the negative stuff about Sonny Bill Williams".

"There's no salary cap on workers, for a plumber or a carpenter or a builder," Mundine added.

"A worker should get what he's worth. There should be no salary cap.

"There should be no salary cap and they (players) should get what they're worth."

The Bulldogs, in conjunction with the NRL, began court action against the renegade star on Monday with the NSW Supreme Court issuing a subpoena for Williams to appear in court next Tuesday.

Williams is believed to be currently en route to Toulon for talks with the French rugby club about a possible switch to rugby union.

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