WASHINGTON, Jan 21 (Reuters) - A last-minute bid by the outgoing chief federal communications regulator to deepen a probe of Comcast Corp CMCSA.O is fueling the hopes of those pushing for unfettered access to the Internet.

The Federal Communications Commission said this week that Comcast’s attempt to revise the practices it uses to manage Internet traffic unfairly favors its own voice Internet service.

The last day at the FCC of Kevin Martin, the outgoing Republican chief, was Tuesday, which leaves the issue to his successor. Democratic sources say the new chairman will be technology executive Julius Genachowski, who advised President Barack Obama on his technology agenda during the transition period.

“The president has long been a supporter of net neutrality; his positions are clear,” said Ben Scott, policy director of the group Free Press, which urged the FCC to look into Comcast’s practices. “And Genachowski was the driver behind (Obama’s) technology platform.”

In a precedent-setting decision last year, the five-member FCC voted 3-2 to uphold a complaint accusing Comcast of violating the FCC’s open-Internet principles by blocking file-sharing services, such as those that distribute video and television shows.

“What this represents in my view is the commission taking seriously what it started with its investigation of Comcast,” Scott said.

The case became a flash point in a debate over a concept known as “network neutrality,” which pits open-Internet advocates against some Internet service providers, which say they need to take reasonable steps to manage ever-growing traffic on their networks for the good of all users.

Comcast, which is appealing the FCC’s earlier decision, has since revised its practices. But the FCC said in a letter this week that the company is still discriminating, this time in how it treats voice over Internet protocol, or VoIP, calls.

The FCC wants “a detailed justification for Comcast’s disparate treatment of its own VoIP service as compared to that offered by other VoIP providers on its network,” agency staff wrote in a letter dated Jan. 18.

A spokeswoman for Comcast could not be reached for comment.

The FCC has guidelines to prevent Internet service providers, like Comcast or telephone companies AT&T Inc T.N and Verizon Communications VZ.N, from discriminating against certain network traffic.

Backers of net neutrality want teeth behind those guidelines and some back legislation to codify them.

Andrew Schwartzman, president of the nonprofit Media Access Project, said that other Internet Service Providers (ISPs) are watching the FCC and the court decision on Comcast’s appeal.

“So far as we can tell, the only ISP which tried to use this highly questionable technique was Comcast,” he said. “But they all would like to do it -- they are watching this case.” (Reporting by Kim Dixon, editing by Matthew Lewis)