Xonjenese Jacobs, right, helps Kristen Niemi sign up health coverage at the University of South Florida. (Chris O'meara/AP)

The White House on Thursday ordered federal health officials to immediately halt all advertising and other outreach activities for the critical final days in which Americans can sign up for 2017 health coverage through Affordable Care Act marketplaces.

The directive stunned some staffers within the Department of Health and Human Services, according to a source close to the department who was briefed on the action. Staff members protested to Trump appointees at HHS that the sudden ban on outreach would suppress the enrollment of the most desirable customers — younger, healthier people who tend to buy coverage at the last minute — which in turn could raise insurance prices in the future, the source said.

An HHS spokesman declined Thursday night to confirm the directive, saying only that HealthCare.gov, the website for the law’s federal insurance exchange, was continuing to accept sign-ups. A federal call center for ACA marketplaces was also still operating, and there was no indication that either would be dismantled before the open-enrollment period’s scheduled end on Tuesday.

Three other sources described the written message, speaking on the condition of anonymity about a decision that the new administration has not announced.

One source said that the White House instructed the Centers for Medicare and Medicaid Services, which oversees much of the ACA’s implementation, to withdraw all communication contracts, marketing plans and advertising set for between Thursday and the end of January.

According to the sources, the move included a halt to ads that the government already had bought. Before the Obama administration ended with Trump’s inauguration, federal health officials paid for several million dollars’ worth of ads for the enrollment period’s final week to encourage people to sign up for ACA health plans, according to one source, who said it was unclear whether the government could recoup that money.

The White House’s order could hamper one of the two weeks of the enrollment season, which began in November, that have the greatest surge of consumers signing up. In the three years since the ACA marketplaces began, the final deadline for enrollment has been the second-biggest day, topped only by the December deadline for people seeking coverage effective Jan. 1.

Research has shown that the last week of open enrollment tends to draw younger enrollees — an important group, underrepresented in the ACA marketplaces, whose scant use of medical services can help keep insurance prices lower for everyone in a given health plan. For the last week of the sign-up period, HHS had planned a major advertising campaign specifically aimed at that younger cohort, as well as emails and text messages to people who have looked at the HealthCare.gov site in the past but not signed up.

On Jan. 10, Obama administration health officials announced that 11.5 million people had signed up for — or been automatically re-enrolled in — ACA coverage by Christmas Eve in the federal exchange on which 39 states rely or similar marketplaces run by the remaining states and District of Columbia. That figure was slightly ahead of a year ago.

The halt to these enrollment efforts comes after President Trump signed a broad executive order just hours after his inauguration last week, instructing federal agencies to soften or eliminate unspecified federal rules to ease the “cost, fee, tax, penalty or regulatory burden” the ACA places on consumers, the health-care industry and medical professionals. No rules have been altered yet, though, and Thursday’s ban on enrollment outreach activities was the administration’s first concrete action to undercut the law it has vowed to abolish.

As word of the directive seeped out late Thursday, it triggered an immediate backlash from former health officials from the Obama administration and other ACA proponents.

“It’s outrageous. It’s irresponsible,” said Ben Wakana, a senior HHS spokesman until Trump took office. “They are deliberately undermining open enrollment to try to get a lower enrollment number.”

Ron Pollack, executive director of Families USA, a consumer health lobby, wrote in an email: “The Trump administration’s mean-spirited decision to pull the already-paid-for enrollment ads belies the president’s promise that he wants to cover ‘everybody’ with health insurance.”