Allegations that Verizon contractors have used undocumented workers have emerged as the major stumbling block to approval of an agreement that would allow the communications giant to compete against Comcast for city cable customers.

Yesterday a key union wrote to Pittsburgh City Council, urging them to compel Verizon to agree to outside auditing of its contractors' work forces. Today labor leaders will meet with Mayor Luke Ravenstahl to plead their case.

The letter from Laborers District Council of Western Pennsylvania President Philip Ameris to council members called Verizon a "chief violator for using undocumented illegal labor."

"We are in favor of Verizon getting approval for the installation of the FiOS [TV] system in this area," said a union spokesman. "However, in regard to Verizon's application, we encourage City Council to require that Verizon not employ directly or use any contractors which would employ illegal workers on the project."

Verizon spokesman Lee Gierczynski said the company "holds its contractors to the highest standards regarding the employees" they hire. The company has given council a letter outlining its policies, which require that contractors hire immigration attorneys or consultants to check their employment rolls. The letter says that in Plum Borough in 2007, when contractors were found to be violating immigration law, Verizon worked with authorities and the municipality to stop it.

Allegheny County District Attorney Stephen A. Zappala Jr. said his office has looked into labor or immigration law issues involving Verizon contractors in Plum, South Park and Ross, but has found it hard to press charges.

"We go out, and [investigators] start asking questions, and they say, 'We'll get you some paperwork,'" he said. "And then the people on the job site change, or the contractors leave the area.

"We have ongoing matters," he said, refusing to elaborate on current investigations.

The Laborers District Council wants Verizon to work with the union-backed Foundation for Fair Contracting to audit the work forces of its contractors on FiOS installations.

"Asking Verizon to commit to hiring a new immigration auditor is beyond the scope of this agreement" to allow cable service in the city, said Mr. Gierczynski. "It's unfortunate that a vote to bring cable competition to the city has been delayed."

Council members seemed split on the issue yesterday, and postponed a tentative vote on the proposed TV pact.

"I believe that the vast majority of the citizens of the city of Pittsburgh want competition in their cable providers," said Councilman Ricky Burgess.

But Councilwoman Darlene Harris said union concerns must be addressed.

"Even though Verizon is back there shaking its head no, I would hope that Verizon would sit down and try to work out these issues," she said, looking at a clump of Verizon executives and backers sitting in council's public benches.