The first phase of the Metrorail extension to Loudoun County could be nine days behind schedule — or six months — depending on who’s giving estimates of when the project will be completed.

Officials with the Metropolitan Washington Airports Authority, which is supervising construction of the line, said Wednesday that the project’s completion date has slipped from July 31, 2013, to Aug. 9, 2013. Upon completion, the line is to be turned over to Metro to operate and maintain.

“The schedule has been changed,” said Jack Potter, chief executive of MWAA. “We had more days where we couldn’t construct because of weather.”

The 11.5-mile first leg is under construction from Falls Church through Tysons Corner to Reston. Officials have said that portion of the extension should be ready for riders by December 2013, following tests by Metro.

But Dulles Transit Partners, the lead contractor on the $2.8 billion project, estimated in a progress report last month that the delay was far more: 188 days.

MWAA officials dispute that estimate.

“They play games trying to make the case for us holding them up,” said Pat Nowakowski, executive director of the Dulles rail project.

He said MWAA is in dispute with Dulles Transit Partners, led by the construction and engineering firm Bechtel, over an electrical substation at Hunter Mill Road that will help power trains. The substations were supposed to be installed in sequence along the rail line, but crews found a gas line in the way at Hunter Mill, he said.

Instead of skipping it and moving along to the next one, Nowakowski said, Dulles Transit Partners is projecting that it will take substantial time to address the substation problem and has added that to the project.

But Leslie N. Pereira, spokeswoman for Dulles Transit Partners, said the contractor “is confident that construction of the Dulles Corridor Metrorail Project can be completed on schedule. We continue to work cooperatively with MWAA to develop and agree on an appropriate mitigation plan.”

Pereira said the contractor works to identify early “issues that have the potential to impact schedule or cost.” The parties then work together on a solution, she said, a process that “has proven very successful” on the Dulles project.

However, Hill International, hired by the federal government to provide oversight, has described the negotiations between Dulles Transit Partners and the airports authority as “difficult and protracted.” It called some of Dulles Transit Partners’ schedules “a tactical posturing move by DTP to argue costs.”

MWAA board members expressed concern during a meeting Wednesday about the dispute with the contractor.

Mame Reiley, chair of the MWAA committee for the Dulles rail project, said she understands some delays, but she is concerned about the negotiations to get the project on track.

“It’s been going on for so long,” she said. “It really needs to come to some settlement.” She said it is “imperative” the disputes be resolved before bidding starts on the second phase of the project. Local, state and federal officials are still negotiating how to pay for the second phase, which will stretch from Wiehle Avenue to Dulles International Airport and Route 772 in Loudoun County.

“This has been dragged out for quite some time,” Reiley said. “Put it in high gear.”

Construction of the line isn’t the only factor that will affect service. Metro has to test all of the equipment. And new rail cars intended to serve the line are being designed and built on their own schedule.

Nowakowski said MWAA will test the system before it relinquishes control to Metro, and there could be delays “if the testing doesn’t get done or [Metro finds] something.”

Metro spokesman Dan Stessel said the transit authority’s tests will involve “running non­revenue trains and ensuring that all components of the line — switches, signals, power, track, structures, et cetera — are all working as intended.” The testing also allows for the “training of operators on the new segment of the system,” he said.

The report says the “preliminary design reviews” of the new 7000 series rail cars, which are being made by Kawasaki, are expected to be done by the end of this month — six months past the scheduled date of May 3.

But Metro officials said they are on schedule for the rail cars to be delivered at the end of 2013. If they are not, Metro said it is prepared to run older rail cars on the line.

The National Transportation Safety Board, however, has urged Metro to replace its oldest rail cars, the 1000 series, saying they are old and not as safe.

Stessel said two officials from Metro recently visited Japan to check on the status of the rail cars, which are being made there and will be assembled in Lincoln, Neb.

MWAA officials have said Kawasaki had experienced some delays in its operations because of the tsunami in Japan this year.

But Stessel said Metro is “monitoring the situation” and expects to have a “better sense of what, if any, impact there may be closer to the end of the year.”

Potter told board members Wednesday that there are “always issues” in building large projects but assured them the first phase was on track.

“As of today we believe we are on schedule to finish on budget, however, that statement is made with a lack of knowledge of unforeseen circumstances,” he said.

“No one knows what the weather will be. We are managing the project on budget and on time.”