A dispute by a Riverside environmental group and transportation planners appears headed for the courtroom, potentially delaying a long-sought extension of Metrolink service to Perris.

In a statement, the Riverside County Transportation Commission said Thursday that negotiations between the agency and Friends of Riverside Hills over the environmental report for the Perris Valley Line had ceased. The project would extend Metrolink service from downtown Riverside to Perris via a 24-mile section of track owned by the transportation commission.

Friends of Riverside Hills filed a lawsuit in August challenging the environmental report for the rail project, saying officials did not adequately detail the noise and pollution impacts of excavating dirt around the tracks and other aspects of construction.

Richard Block, a Riverside Hills member, said concerns ranging from the tons of dirt that will be hauled away from the project to the squealing of train wheels are unresolved.

“The evidence presented in the environmental process clearly shows the project will have negative impacts,” Block said. “And they are either incompetent to them or ignoring them.”

He also said officials are grossly overestimating the line’s ridership, noting that the 4,300 daily riders estimated in the report would exceed the ridership of existing Metrolink service in Riverside County. Ridership along the entire 91 Line from Riverside to Union Station in Los Angeles was 5,161 in June, according to Metrolink.

Officials stand by the environmental assessment and claim the mitigations they have detailed are satisfactory, said John Standiford, the commission’s deputy executive director.

Block and Standiford both said they could not detail specifics of the negotiations. But in a news release, transportation officials confirmed the dispute was destined for trial, delaying construction.

“The Perris Valley Line Metrolink extension will provide an environmentally friendly commute option by utilizing an existing rail track that has been in the area for more than a century, thus avoiding significant environmental impacts,” Supervisor John Benoit said, in the news release.

Officials have said the cars the project will take off I-215 cause far more pollution than the rail line extension.

Litigation would further delay the project which was initially supposed to start construction in late spring, based on estimates last year. Now the work will wait until the lawsuit is resolved. Standiford said officials would hope a court decision could come by late fall. Officials would have to wait on other approvals and award a contract for the construction, probably in early 2013.

Based on estimates, service on the line would start about 18 months after construction begins. Transportation officials say the project would create more than 4,000 jobs.