Any challenge by runaway rugby league star Sonny Bill Williams to the NRL's salary cap rules is doomed to failure, according to legal opinion.

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

But a Victorian Bar spokesman in Melbourne said it was unlikely such a bid could succeed.

The deputy chairman of the sports section of the Victorian Bar, Tony Nolan, said courts were more concerned with how reasonable the rules were rather than whether they were technically illegal.

"All restraints are illegal in some sense," Nolan told Fairfax Radio. "The test is not the illegality, it's the reasonableness of the restraint.

"The AFL has argued for some considerable time that the twin pillars of restraint of trade, together with the salary cap in relation to their draft, have acted to ensure the evenness of the competition."

He said the premise of the AFL and NRL was to keep their competitions even and to ensure the survival of weaker clubs.

"Nolan said if Williams were to win an action in court which resulted in the cap being scrapped and players could be paid whatever clubs were prepared to pay them the platform of the sports would collapse.

"His restraint is, I want to cut out the financial provisions so that clubs can sign up whoever they want," Nolan said.

"It would bankrupt half the clubs in rugby league who are in financial trouble already if the player demands increased by two or three or four-fold overnight."

He said the salary cap was not "illegal unless it's unreasonable".

Nolan said technically even a normal contract between an employer and employee was a restraint of trade.

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