People may have also mistakenly believed they did not have to comply with the law’s reporting requirements. The new guidance suggests taxpayers will now face a sharp reminder that they need to provide this information, when they go to file a return electronically or submit the appropriate paperwork to get any refund they are due.

Under the law, an individual who does not have insurance can face a penalty of $695 a year for an individual, up to a maximum of $2,085 for a family or 2.5 percent of your adjusted gross income, whichever is higher. People are exempt from the penalty if they have too little income or if the lowest-priced coverage available costs them more than 8.16 percent of their household income.

The I.R.S. had initially held off rejecting returns because the law was new, but then it delayed its plans to assess the effect of the executive order, said Nicole M. Elliott, a tax lawyer for Holland & Knight and a former I.R.S. official involved in putting into effect the Affordable Care Act.

“It is curious, given the executive order,” Ms. Elliott said. “It’s a little unclear where the agencies are going and how heavy-handed they will be in enforcing it.”

In evaluating its stance, the agency may have decided the requirement eases the burden on taxpayers by making it clear they need not worry if they have insurance or are exempt from the penalty, she said.

But the I.R.S. may still decide not to actively enforce the mandate, Ms. Elliott added. While the agency is taking steps to be sure it collects all the information necessary to levy the penalty, it could also take a very lenient view of how aggressively it goes after anyone who does not sign up. “It’s dangerous to read too much into this,” she said.

The White House declined to comment.

The I.R.S.’s decision to actively prepare to enforce the mandate only adds to the uncertainty about where the president stands about the future of the law. Just this month, he issued a second executive order aimed at allowing the sale of skimpier plans to both individuals and small businesses, the same day he announced he would abruptly stop funding subsidies for low-income individuals. He has abruptly switched positions on a new bipartisan proposal aimed at providing short-term stability to the insurance marketplaces under the law.