WASHINGTON — The Obama administration on Wednesday announced a one-year delay in a major element of the new health care law that would allow small businesses to buy insurance online for their employees through the new federal marketplace.

It was yet another setback for the rollout of the health care law and resulted, in part, from the well-documented problems of the insurance marketplace website. Administration officials said they had to focus on the basic functions of the website, so that individuals could shop for insurance, before offering online enrollment for small businesses. In the meantime, businesses and their employees can apply through brokers.

Many employees of small businesses are uninsured, and the businesses themselves are much less likely than big companies to provide coverage to workers and their families.

The latest delay, coming just as the White House was boasting of major improvements in the health insurance website, HealthCare.gov, opens the door to more complaints about the health care law and could increase pressure to delay other provisions.

“The president bit off more than he can chew with this health care law, and small businesses are now forced to bear the consequences,” said Speaker John A. Boehner of Ohio. “Business owners across the country are already having health care plans for their employees canceled by this law, and now they’re told they won’t have access to the system the president promised them to find different coverage. Instead, they’ll have to resort to a system you’d expect to see in the 1950s.”

It was not the first delay for small businesses. The administration had previously delayed online enrollment for them to the end of this month from Oct. 1.

The date has now been pushed back to November 2014 for coverage that takes effect in January 2015, according to the Health and Human Services Department.

The announcement, just before Thanksgiving, was reminiscent of the way the White House announced, just before the Fourth of July weekend, a one-year delay in the requirement for larger employers to offer health insurance to employees.

The marketplace for small businesses — the Small Business Health Options Program, or SHOP exchange — was one of the few provisions of the 2010 law with some Republican support, and it was originally championed by Senator Olympia J. Snowe, Republican of Maine.

John C. Arensmeyer, the chief executive of Small Business Majority, an advocacy group that supports the health care law, said, “It’s disappointing that the online portion of the federal small business marketplace through Healthcare.gov will be delayed, and it’s important it get up and running as soon as possible.”

The marketplace, he said, “is still the most important provision in the Affordable Care Act for small businesses.”

For years, small businesses have had difficulty getting affordable insurance. Many owners cite the rising cost of health insurance as their top concern.

The administration said that small businesses and their employees seeking coverage in the federal exchange could still apply and enroll through an agent or broker, as many do now. “Agents and brokers are essential to making this happen,” an administration official said.

However, the high-tech capability once promised by the White House will not be available until late next year.

“The agent, broker or insurer will help the employer fill out a paper application for SHOP eligibility and send it in to the SHOP marketplace,” the administration said. The insurer can also tell employers what premiums they would have to pay and can enroll employees.

Some small businesses may qualify for tax credits worth up to 50 percent of their premium costs. The tax credits will be available only for plans purchased through the small business exchange.

Amanda L. Austin, a lobbyist at the National Federation of Independent Business, a trade group, said she had heard rumors that the online small business exchange might be delayed, but was surprised that it had been put off for a year. “That’s pretty significant,” Ms. Austin said. “The online exchange is a key component of the Affordable Care Act, and administration officials have hailed it as the answer to small businesses’ health care concerns.”

While the online exchange is being delayed, she said, “many small businesses face higher premiums in 2014 because of new taxes, including a new federal tax on the health insurance they purchase.”

Kathleen Sebelius, the secretary of health and human services, said that the website was “vastly improved each and every day,” with hardware upgrades and software fixes that produced lower error rates and faster response times for users.

An employer using the SHOP exchange must offer coverage to all full-time employees — generally those working at least 30 hours a week, on average.

In April, the Obama administration delayed a requirement that SHOP exchanges offer a variety of competing insurance plans to employees. The administration cited “operational challenges” as a reason for that delay.

Congress had wanted to provide small business employees with a range of health plan options. While some state-run exchanges will allow employers to offer such choices to employees, the federal exchange will not do so until 2015.

E. Neil Trautwein, a vice president of the National Retail Federation, said, “If the law is so burdensome for the administration to implement, just think how hard it is for small businesses.”

In a separate announcement, officials at the Health and Human Services Department said Wednesday that they would replace the contractor that manages computer servers handling the enormous load of data collected by HealthCare.gov.

Terremark, a subsidiary of Verizon Communications that now provides the service, will be replaced in March by Hewlett-Packard, officials said. A spokeswoman at the Centers for Medicare and Medicaid Services said the change had been planned at least since July. Issues related to the raw computing capacity of the servers provided by Terremark have been cited as a factor in the website problems. A spokesman for Verizon declined to comment.

The problems engulfing President Obama’s health care law are remarkable because administration officials had repeatedly brushed aside doubts about whether they would be ready.

Testifying before a congressional panel on Oct. 29, Marilyn B. Tavenner, the administrator of the Medicare agency, said, in response to a question, that the website for the small business exchange would be in operation by the end of this month.