A federal judge Monday said his court order holding up construction of Maryland’s Purple Line will remain in effect while the state continues to fight a lawsuit opposing the light-rail project.

In a 12-page ruling, U.S. District Judge Richard J. Leon rejected the state’s plea to reinstate the project’s federal environmental approval, which Leon revoked in August. Construction on the 16-mile Purple Line can’t begin — and the state can’t secure $900 million in federal grants — until a court restores that approval.

Leon said the state’s lawyers did not meet two “stringent requirements.” The state, he said, did not show “a likelihood of success” in appealing part of the lawsuit that it lost and did not prove it would suffer “irreparable harm” if the project is further delayed.

Leon rejected the state’s arguments that legal delays are costing taxpayers $13 million a month and put the project at risk of being canceled, which would cost millions in contract termination penalties. Leon noted that the state signed a $5.6 billion public-private partnership after the plaintiffs asked him to suspend the project’s environmental approval and continued to spend money on pre-construction work amid legal uncertainty.

“Maryland was explicitly on notice that plaintiffs were asking me to vacate the [environmental approval] and could have structured its public-private arrangements to better protect its financial interests” in case the environmental approval was suspended, Leon wrote.

Leon’s ruling probably will have little effect on the speed of the legal case because Maryland officials have asked an appeals court to decide the same issue. The U.S. Court of Appeals for the D.C. Circuit has yet to rule on whether the environmental approval should be reinstated during the lawsuit’s appeal.

The state is appealing Leon’s August decision that the ridership forecasts done as part of the Purple Line’s environmental study failed to adequately account for Metro’s safety problems and declining ridership. The Purple Line would be separate from Metro, but 27 percent of its riders are expected to use it to reach Metro stations.

Federal transit officials have said studies show that the Purple Line’s ridership would be “one of the most robust” among U.S. light-rail systems, even if no Metro riders used it, and that ridership would not affect the rail line’s environmental ­footprint.

The plaintiffs have said the rail line should not be built until questions surrounding its potential environmental impacts have been resolved. Starting construction, they say, would destroy trees, wildlife habitat and other environmental resources.