Filing even the simplest of returns could pose challenges during a pandemic. The I.R.S. does have a free filing site, but those who lack internet access could be unable to use it because nonprofits, libraries and other places are closed. Low-income filers, however, will have until the end of the year to file and still get their stimulus payments.

The decision to require more people to file returns puzzled policy experts, who said the government had the ability to crosscheck various databases to make sure it reached everyone who was eligible, including those who would have the greatest need.

“The matching part may take longer, and it’s legitimate for them to say that one group will get payments faster and one group will be slower,” said Ms. Parrott, who worked in the federal Office of Management and Budget during the Obama administration. “But to throw up their hands and say they can’t do it? They have the capacity to do it, and I think they need the leadership to say that they are going to get this done.”

There will be an extra layer of difficulty for people who don’t normally file a return and lack a standard checking account.

The Financial Health Network, a research and advisory group, estimated that 12 million people earning less than $30,000 per year were among the so-called unbanked. But the guidance issued Monday says people who are filing returns just to receive their stimulus payment must supply direct-deposit information.

“There are a surprisingly large number of people who are unbanked in the U.S.,” said Michael Graetz, a Columbia University law professor and former deputy assistant secretary for tax policy at the Treasury Department. “They cash their checks at check-cashing facilities and do lending through credit cards or payday loan operations.”

Many people who don’t maintain a bank account have relationships with financial services companies that help distribute prepaid debit cards for various forms of government aid, like the Supplemental Nutrition Assistance Program.

Companies, like Conduent and FIS, that help run such programs possess information for many people who are eligible for the recovery payments but do not pay taxes or receive Social Security. The companies declined to comment on whether the Treasury had approached them about a partnership; the department also declined to comment.

Mr. Mnuchin has said he expected automatic payments to begin arriving this month. But the I.R.S. guidance will almost certainly mean longer waits for those who must file new returns to be eligible to receive a stimulus payment.

Policy experts worry that payments will be slow to reach those who need them. But the bigger fear is that some eligible people might not get payments at all.

When the federal government established a stimulus program in response to the 2008 financial crisis, at least 3.4 million people who were eligible failed to file the necessary tax return in order to get it, according to Treasury Department data. And that was for a smaller payment than the one on offer now — and a smaller group of eligible citizens.

By Ron Lieber and Alan Rappeport, New York Times

Shira Ovide, Contributed Reporting