The process to start building the second phase of Metro’s new Silver Line is on hold while Loudoun County officials decide whether to support the project, according to the authority overseeing construction.

The request for contractors to submit price quotes was supposed to go out in early March, with bids being accepted by the end of the year and a winner being chosen by early 2013, according to officials at the Metropolitan Washington Airports Authority. Construction would start in the spring of 2013 and the project would take four to five years to complete.

But the requests haven’t gone out, as MWAA waits to see if Loudoun is in or out on the deal.

Loudoun has until July 4 to decide whether it is putting up its share — 4.8 percent — of the estimated $2.8 billion construction cost on the second phase of the Silver Line, which is planned to run from Reston to Dulles International Airport and into Loudoun.

Supervisors in Loudoun County had asked for more time to review the details of the Silver Line because the board underwent major turnover after the November elections and many of the officials have concerns about the project’s costs. Loudoun also is expected to contribute about $11 million in 2018 — the first full year the second phase of the rail line will run — for its share of operations.

Construction scene at the Dulles Airport Metro extension project (also known as the Silver Line) near the intersection of Rt. 7 and Spring Hill Road in the Tysons Corner area of McLean. (Tracy A. Woodward/THE WASHINGTON POST)

In addition, Virginia had promised to put up $150 million for the second phase of the project, but that commitment has been stalled over concerns about a provision favoring union labor on the project.

“Everything is so iffy,” said Patrick A. Nowakowski, executive director of the Dulles rail project at MWAA. “If Loudoun pulls out, we’re back to the drawing board. The funding agreement falls apart.”

The Silver Line has several partners that are paying a share of the second phase of the project’s cost. MWAA’s share is 4.1 percent; Loudoun is supposed to be putting up 4.8 percent; Fairfax County is putting up 16.1 percent; and the rest comes from Dulles Toll Road users who will have to pay higher rates. Money from Virginia and federal grants programs would potentially help to lower the tolls drivers pay.

Meanwhile, MWAA officials said the first phase of the project, which runs through Tysons Corner to Reston, will likely finish $150 million over budget because of moving utility lines, track installation and changes sought by Metro to comply with safety recommendations from the National Transportation Safety Board. Officials said they had hoped to make up the cost overruns over time, but some bids have come in higher than budgeted.

The first phase is expected to be completed in August 2013, but it wouldn’t open for passengers until December 2013 or early 2014. MWAA is overseeing construction of the line, but Metro will operate it.