Former welterweight titlist and longtime pound-for-pound ranked fighter Paul Williams, making a comeback from a crushing second-round knockout loss to middleweight champ Sergio Martinez in November, was due to return July 9 against Japan's Nobuhiro Ishida in the main event of HBO's "Boxing After Dark."

However, after approving the junior middleweight fight -- which was met with harsh criticism as not being worthy of being a main event on the premium cable service -- HBO withdrew its approval on Tuesday, according to multiple sources involved in the event.

Dan Goossen, Williams' promoter, had negotiated the bout with Golden Boy, Ishida's co-promoter, and both sides told ESPN.com the fight had been agreed to, approved by the network and that they were only waiting for Ishida's signed contract to be returned.

However, HBO pulled its approval without explanation just days after giving it the go-ahead.

Goossen declined comment.

The search is now on for a new opponent for Williams (39-2, 27 KOs), whom trainer George Peterson said has already been in training camp for three weeks.

HBO had accepted Ishida as the opponent after the Williams camp turned down a number of proposed opponents, including former titlist Sergiy Dzinziruk and former title challenger Deandre Latimore.

Ishida (23-6-2, 8 KOs) gained notoriety on April 9 when he scored perhaps the biggest upset so far of 2011, knocking out comebacking contender James Kirkland (in his third fight since being released from prison) in the first round of a middleweight bout on the Marcos Maidana-Erik Morales HBO PPV undercard in Las Vegas.

At first, Williams-Ishida was slated to headline the network's flagship series "World Championship Boxing" before being downgraded this week to "B" series "Boxing After Dark" and then canceled altogether.