Artist rendering of the planned Purple Line commuter trains in Maryland. (Purple Line Transit Partners)

Maryland officials who had expected to clinch $900 million in federal funding Monday to build the Purple Line will have to wait after a court ruling made the light-rail project temporarily ineligible for federal aid.

The Federal Transit Administration postponed the Monday signing event indefinitely, according to an email Thursday from the Maryland Department of Transportation to people who had been invited to the ceremony.

MDOT had planned to publicly enter into a funding agreement Monday with the FTA, which would have locked in nearly half of the $2 billion project’s construction funding. Such agreements are the result of a years-long quest for highly competitive federal transit aid and are typically signed amid great fanfare.

But a ruling Wednesday by U.S. District Judge Richard J. Leon in Washington made the light-rail project ineligible, temporarily at least, for federal money, transit experts said. The judge ordered that the project’s “record of decision” — the federal stamp of approval on the state’s environmental impact study — be vacated, or set aside, until the state recalculates the Purple Line’s ridership forecasts to reflect Metro’s falling ridership.

The Purple Line will be independently owned and operated and is not part of the Metro system. However, passengers transferring to and from Metro are estimated to make up 27 percent of Purple Line riders.

An FTA spokesman confirmed that the financial award had been postponed because the judge’s order left the project without a required, valid stamp of approval.

The signing of the Full Funding Grant Agreement was considered the last major hurdle before Purple Line construction could begin on schedule late this fall. Press offices for the MDOT and the MTA did not respond to questions about how the funding delay could affect that timing.

Maryland transportation officials said only that state Transportation Secretary Pete K. Rahn asked the FTA Thursday to appeal the judge’s ruling. They referred all questions to the Justice Department.

“We are very limited in what we can say, given this is active litigation,” MDOT spokeswoman Teri Moss wrote in an email.

The 16.2-mile line will have 21 stations between Bethesda in Montgomery County and New Carrollton in Prince George’s County, including in Silver Spring, Langley Park and the University of Maryland’s flagship College Park campus.

Federal and state lawyers had said in a June court hearing that redoing the ridership calculations could delay the project by six months and jeopardize its highly complex, $5.6 billion public-private partnership that relies heavily on private construction financing.

Delays in major construction projects typically raise costs, and lawyers for the state had argued that even a “modest delay” could have “cascading consequences on the project schedule and financing arrangements.”

Whether the private partner is having second thoughts after the funding delay is unclear. Jamie Breme, a spokeswoman for Purple Line Transit Partners, the team of companies that recently signed a 36-year contract on the project, referred questions to MDOT.

“At this time, we’re working with our client,” Breme said.

The court order stemmed from a 2014 lawsuit, brought mostly on environmental grounds. In the suit, Purple Line opponents argued that the ridership projections used to justify the cost of building a light-rail line rather than cheaper transit options were flawed because they did not reflect Metro’s decline in passengers. The plaintiffs — two Chevy Chase residents and the nonprofit group Friends of the Capital Crescent Trail — noted that Metro’s ridership has fallen steadily since a 2009 peak and that some passengers have avoided the system because of high-profile safety lapses and near-daily breakdowns.

Lawyers for the state argued that Metro’s problems would not affect the Purple Line because it will be operated separately from Metro and with different technology. Federal lawyers argued that the FTA is “keenly aware” of Metro’s problems and that Metro is making “substantial investments” in its system via the SafeTrack repair program.

Federal lawyers also argued that Metro’s falling ridership will not be an issue in 2040, the year of the Purple Line’s official ridership forecast.

Art Guzzetti, of the American Public Transportation Association, said the Purple Line is getting national attention because of its unusual public-private partnership. It is one of the first transit projects in the country to rely on private construction financing.

Guzzetti said he could not comment on the judge’s ruling but said transit ridership typically is viewed in the context of long-term trends, such as population growth and development likely to occur around stations.

“Metro’s ridership is down now, but over time it’s been very strong, and the long-term trend is very strong,” Guzzetti said. “You don’t look at short-term blips.”

Greg Sanders, vice president of the advocacy group Purple Line Now, called the cancellation of the funding-agreement signing a “significant complication” that will no doubt delay construction.

“We’ve been at this for a long time,” Sanders said. “The fundamental appeal of the Purple Line is that it provides a needed east-west transit alternative. Any other problem doesn’t change that need.”