Copies of the application for the Health Insurance Marketplace from the Department of Health and Human Services are distributed. (Andrew Harrer/Bloomberg)

The Obama administration has decided to give extra time to Americans who say that they are unable to enroll in health plans through the federal insurance marketplace by the March 31 deadline.

Federal officials confirmed Tuesday evening that all consumers who have begun to apply for coverage on HealthCare.gov, but who do not finish by Monday, will have until about mid-April to ask for an extension.

Under the new rules, people will be able to qualify for an extension by checking a blue box on HealthCare.gov to indicate that they tried to enroll before the deadline. This method will rely on an honor system; the government will not try to determine whether the person is telling the truth.

The rules, which will apply to the federal exchanges operating in three dozen states, will essentially create a large loophole even as White House officials have repeatedly said that the March 31 deadline was firm. The extra time will not technically alter the deadline but will create a broad new category of people eligible for what’s known as a special enrollment period.

The change, which the administration is scheduled to announce Wednesday, is supported by consumer advocates who want as many people as possible to gain insurance under the 2010 Affordable Care Act. But it’s likely to be criticized by Republicans who oppose the law and have denounced the way the administration is implementing it.

The ad for HealthCare.gov features Jonah Hill, Adam Levine, Jennifer Lopez and Alicia Keys' moms. (The Washington Post)

Administration officials said the accommodation is an attempt to prepare for a possible surge of people trying to sign up in the final days before the deadline. Such a flood could leave some people unable to get through the system.

“We are . . . making sure that we will be ready to help consumers who may be in line by the deadline to complete enrollment — either online or over the phone,” said Julie Bataille, director of the office of communications for the Centers for Medicare and Medicaid Services, the agency overseeing the federal health-care exchange.

The extra time will not be restricted, though, to people who wait until the last minute to try to sign up. Although no one will be asked why they need an extension, the idea is to help people whose applications have been held up because of the Web site’s technical problems, or who haven’t been able to get the system to calculate subsidies to help them pay for coverage.

According to a Health and Human Services official, who spoke on the condition of anonymity about decisions that have not been made public, an exact time frame for this extension has not been set, and it will depend in part on how many people request it. Nor have officials decided precisely how long people will have to select a health plan after they get the extra time.

Starting in about mid-April, people will no longer be able to get extensions through HealthCare.gov. After that, consumers will be able to request one through one of the federally sponsored call centers nationwide. At that point, the grounds for an extension will become narrower, matching rules for special enrollment periods that have existed for the past few months. Those include people who have a new baby, are getting a divorce, lose a job with health insurance or had a technical problem signing up for coverage through HealthCare.gov.

Once the narrower rules take effect, people will still be trusted to tell the truth about why they need more time — a method known as “self-attestation.”

The new rules are similar to steps being taken — or contemplated — by some of the 14 states that are running their own health-insurance exchanges.

Last week, the governing board of Maryland’s exchange, which has been hampered by serious computer problems, decided to let residents complete their enrollments after the March 31 deadline, as long as they had started the process beforehand. Minnesota officials announced this week that they would do the same thing. Oregon’s governor plans to announce a similar plan this week, according to his spokeswoman, and the board of Nevada’s exchange is considering several alternatives, including a special enrollment period.

The impact of such leeway, coming in the final days of a sign-up period that began in October, remains unclear. This year’s enrollment period is the first opportunity for Americans who are unable to get affordable insurance through a job to choose whether to enroll in one of the plans offered under the 2010 law.

In recent weeks, the White House and its allies have been mounting an intense public relations campaign to motivate people to sign up. Amid signs of increasing interest, federal health officials have privately worried whether HealthCare.gov could withstand an expected last-minute enrollment surge this weekend.

Administration officials have adopted an enrollment target of 6 million Americans, forecast this winter by congressional budget analysts. The analysts had lowered their previous prediction of 7 million after problems with HealthCare.gov thwarted many people who attempted to enroll during the fall.

Until now, the March 31 deadline has been the date by which most Americans must choose a plan — or risk a government fine in the form of a tax penalty when they file their 2014 taxes next year. The fine will not apply to people who get an extension under the new rules and enroll in plans within the allotted time.

The constituency that has been most wary of extra sign-up time has been the insurance industry. Insurance firms selling plans in the new marketplace want to minimize the possibility that people might wait to get coverage until they become sick — a practice that would undermine the central idea of keeping costs in check by balancing people who are expensive to insure with those who are healthy and require little medical treatment.

On the other hand, consumer advocates say it is important to give as many people as possible a chance to obtain insurance.

“The whole point of the thing is to get people covered,” said Jon Kingsdale, a health-care consultant and former director of Massachusetts’s insurance exchange, which was the first in the country, opening several years before the federal law set up a similar national marketplace. “In the first year, there has been so much confusion, I think it’s only natural there will be people who just don’t feel as if they fully understood what the law was and what they were supposed to do and that the opportunity would close.”

Jenna Johnson contributed to this report.