The Federal Transit Administration has warned Metro it could face financial hardship if it yields to pressure from the District by restoring late-night service, saying doing so would risk safety by reducing time for maintenance.

In a letter sent this month, the FTA said it may have to postpone certifying a new Metro safety oversight agency if late-night service is extended. That would further delay the lifting of FTA sanctions that have held up millions in federal transportation funds to the District, Maryland and Virginia.

The FTA’s warning adds to pressure on Mayor Muriel E. Bowser (D) and Metro Board Chairman Jack Evans to drop their insistence on extending service hours, especially on weekends. The District says restoring late-night service is vital to support the city’s restaurants, clubs and other nightlife.

Critics contend the shorter hours also hurt late-night workers, many of whom are lower-income residents who have few transportation options.

Bowser shows no signs of relenting. At a meeting Tuesday with Metro General Manager Paul J. Wiedefeld and the D.C. Council, she pointedly urged Metro to come up with new proposals that would both restore late-night service and leave adequate time for maintenance.

“I think we know when an agency doesn’t want to do something, they present the worst possible scenarios,” Bowser said. “So my challenge with Metro is to go back and do some more work.”

Bowser said Metro should be more responsive after the city supported last year’s historic agreement with Virginia and Maryland to provide the agency with an additional $500 million a year in dedicated funding for its capital needs.

“All of us made a substantial commitment to making sure that Metro has what it needs,” Bowser said. “We didn’t go through all of that so we have a system that can’t support our workers, can’t support our businesses and can’t support our people.”

The FTA letter and Bowser’s comments illustrate how both sides are hardening their positions in the conflict over whether Metro should restore late-night service. The service was cut in 2016 to allow crews more track time during the transit agency’s SafeTrack maintenance program. SafeTrack was launched following several critical safety lapses.

Metro would like to maintain its current hours, in which rail service ends at 11:30 p.m. Monday through Thursday, 1 a.m. on Friday and Saturday, and 11 p.m. on Sunday. The agency contends the extra time is critical to give crews adequate opportunity at night for work such as checking electric cables, cleaning track beds and maintaining switches.

But the District has threatened to use a rarely wielded jurisdictional veto on the Metro board if necessary to force a return this summer to the 2016 schedule.

In 2016, rail service ran until midnight Monday through Thursday, 3 a.m. on Friday and Saturday, and midnight on Sunday.

Maryland, Virginia and the federal government support Metro’s position. They also do not want to pay more to cover the cost of providing more service.

“We’re kind of the Lone Ranger on this,” said Evans, who in addition to chairing the Metro board is a D.C. Council member (D-Ward 2).

Metro says that if it returns to the 2016 schedule, it would preserve its maintenance schedule by single-tracking trains during midday, which would slow service. The agency estimates that doing so would cause it to lose 6 million riders in the District and an additional 5 million in the suburbs each year, at a cost of $45 million.

Metro also has proposed two compromise schedules. One of the alternatives, with more single-tracking on weekends, was described by Metro officials as potentially attractive to the District. It would keep the system open until 11:30 p.m. Sunday through Thursday, and until 2 a.m. Friday and Saturday.

Bowser said she was not satisfied with the alternatives.

“Those four options, to me, need a lot of work,” Bowser said. “We will ask our team to look at these numbers more closely about why the only scenarios that you have submitted to us include lost ridership, instead of including better ways that Metro can do this work more effectively.”

Metro spokesman Dan Stessel said the agency was trying to give the city what it wanted.

“The alternatives were developed because the District made it clear they were not happy with the recommendation of safety experts who would prefer a continuation of the current hours,” Stessel said. “The options outlined today attempt to balance D.C.’s request against the need for maintenance time.”

The FTA letter was the latest step by the agency to support stricter safety standards since it took over safety oversight of Metro in 2015. The FTA is supposed to give oversight responsibility to the newly created Metrorail Safety Commission by April 15.

But the handoff could now be delayed, the FTA said, because a reduction in maintenance hours could require the FTA to devote more resources to reviewing various safety improvements that it has asked Metro to undertake. That could also delay FTA approval of the use of federal funds in Metro’s fiscal 2020 budget.

Fenit Nirappil contributed to this report.