The decision by a judge in Texas, which applies nationwide and could have far-reaching implications for millions of Americans, is almost certain to be appealed by the Biden administration.WASHINGTON — A federal judge in Texas who once declared the Affordable Care Act unconstitutional issued a far-reaching ruling on Thursday that prevents the Biden administration from enforcing a provision of the law that provides patients with certain types of free preventive care, including screenings for cancer, depression, diabetes and H.I.V.The decision, by Judge Reed O’Connor of the Federal District Court for the Northern District of Texas, applies nationwide. If it stands, it could have far-reaching implications for millions of Americans, bringing the United States back to the days before the 2010 health law known as Obamacare, when insurers were free to decide which preventive services they would cover.The ruling, which is in the form of a nationwide injunction, takes effect immediately, said Lawrence O. Gostin, an expert on health policy at Georgetown University who has followed the case. It will affect a long list of preventive care services, he said, including services like screenings for heart disease, pap smears and tobacco cessation services.“It might be that tomorrow, a woman might wake up and find that her mammogram is not covered,” Mr. Gostin said, adding, “I think we forget what it was like before the Affordable Care Act, where we had to pay and it was unaffordable for basic primary health care services.”But insurers and employers emphasized that the decision would not have any immediate impact on people’s coverage. “We want to be clear: Americans should have peace of mind there will be no immediate disruption in care or coverage,” Matt Eyles, the president and chief executive of AHIP, a major trade group for insurers, said in a statement.Still, there could be changes over time. Any significant change would require providing advance notice, and employers and insurers would most likely wait until the next plan year to adjust the benefits they provide, if they do at all, according to benefits experts.The Biden administration is almost certain to appeal the ruling and ask for a stay of the injunction. The White House press secretary, Karine Jean-Pierre, said the Justice Department and the Department of Health and Human Services were reviewing the decision.“This case is yet another attack on the Affordable Care Act,” Ms. Jean-Pierre said, adding, “Preventive care saves lives, it saves families money and protects and improves our health.”The lead plaintiff in the case is Braidwood Management; the company’s owner, Dr. Steven F. Hotze, is a well-known Republican donor and Houston doctor who has previously challenged the Affordable Care Act. The plaintiffs argued that a volunteer panel of experts that issues binding recommendations on what preventive care must be covered under the law violated the Constitution because its members are not appointed by the president or confirmed by the Senate.The plaintiffs also singled out drugs that prevent H.I.V./AIDS, arguing that the mandate to cover those medicines violated the Religious Freedom Restoration Act, a 1993 law that prevents the government from imposing a burden on a person’s religious freedom.“This decision introduces uncertainty into an aspect of the health care system that people have benefited from for nearly a decade: access to preventive care with no out-of-pocket costs,” Natalie Davis, the chief executive of United States of Care, a nonpartisan health advocacy group, said in a statement.Ms. Davis said the ruling meant that nearly half of Americans — more than 151 million people — “may lose access to free preventive services, such as mental health, weight loss measures and various cancer screenings that we have all come to depend on.”Advocates for people with H.I.V./AIDS were especially alarmed at the decision.“The fact that a judge in Texas has decided to threaten the health care of all Americans for fringe ideological beliefs is something that should truly scare every American,” said James Krellenstein, a longtime H.I.V./AIDS activist.Judge O’Connor’s ruling was not a surprise. In September, he ruled that the U.S. Preventive Services Task Force — a volunteer panel of experts that recommends what kinds of preventive care must be covered under the Affordable Care Act — violated the Constitution. Thursday’s ruling flows out of that earlier decision.The ruling in September also took explicit aim at the H.I.V. drug regimen known as pre-exposure prophylaxis, or PrEP, saying the law’s requirement that it be covered violated Braidwood Management’s religious freedom. A recent analysis by researchers at Yale and Harvard estimated that for every 10 percent decrease in PrEP coverage among men who have sex with men, there would be an additional 1,140 H.I.V. infections in the following year among that population.Thursday’s ruling comes just a week after the 13th anniversary of President Barack Obama’s signing the Affordable Care Act into law. In 2012, the Supreme Court upheld the bulk of the law but struck down its requirement that states expand Medicaid. In 2018, Judge O’Connor ruled that the entire law was unconstitutional, but the Supreme Court later overruled him.Some experts said that the preventive care benefits mandated by the law were now so ingrained in the American health care system that it would be difficult for employers and insurers to unwind them, and many would choose not to.Even before the mandate went into effect, many employers were providing such benefits to workers, said Beth Umland, the director of research for health and benefits at Mercer, a consultant. The tight labor market and value of those services make it unlikely that employers will want to drop the coverage, she said.“They’re not in a mode to claw back popular low-cost benefits that employees have been used to getting for years,” she said.Some insurers said they planned to continue offering the services at no cost. Cigna Healthcare said its “intention is to continue covering the full range of recommended preventive services without cost sharing for our customers as part of our standard coverage policies, regardless of the final outcome of this court case.”But the lack of a mandate could eventually cause some employers or insurers to cut back on such coverage, particularly for more costly services. It would then be “off to the races, and people will lose some important protections,” said Katherine Hempstead, a senior policy adviser at the Robert Wood Johnson Foundation who has closely followed the Affordable Care Act.Insurers may also wait to see the outcome of various legal actions before changing course. “We expect this will drag on,” said John Baackes, the chief executive of L.A. Care Health Plan, a California insurer. “I don’t think anyone will jump to make any changes until the fog clears.”